Jurassic plesiosauroid Plesiosaurus dolichodeirus with a controversially dipped left hindfin. Nothing like a little drama to start a blog post.

Among the first animals to feature prominently in palaeoart were plesiosaurs, those four-flippered marine sauropterygians that need no introduction to anyone who's reading a blog focused on prehistoric life. Some plesiosaur depictions are among the most spectacular palaeoart of all: their arcing spinal columns, toothy faces and the moodiness intrinsic to seascapes are wonderful ingredients for palaeoartists to play with, leading to two centuries of plesiosaurs as dependably gripping art subjects.

Despite their popularity among artists, the theory we apply to our plesiosaur reconstructions has not been significantly 'modernised' in the way that it has for other prehistoric species, most obviously Mesozoic dinosaurs, pterosaurs or fossil mammals. A number of authors and artists have produced solid foundations for the reconstruction of the latter animals - libraries of skeletal references, assessments of gait and stance, heightened awareness of common soft-tissues, etc. - and their life appearances are now more uniformly reconstructed and prone to fewer obvious errors. This has yet to happen for plesiosaurs, however. Modern skeletal reconstructions are few, references for muscle layout and soft-tissue data are fewer, and discussions over aspects of their life appearance are rare.

I was recently commissioned to produce two studies of two Early Jurassic plesiosaurs - one of the plesiosauroid Plesiosaurus dolichodeirus (above) and another of the pliosaurid Attenborosaurus conybeari (below). I cannot claim any expertise in plesiosaur science, but when reviewing art-relevant literature on these animals it struck me that many familiar elements of plesiosaur palaeoart oppose our soft-tissue data, modern muscle studies and flipper arthrology, as well as the generalities of vertebrate anatomy. I'm sure others have noticed these issues before me, but their prevalence in contemporary plesiosaur art suggests they are not as widely known as they could be. In the interests of stirring conversation on restoring plesiosaurs, I thought I'd share my findings and thoughts here.

Flipper shape and motion

One of the ‘classic’ elements of plesiosaur reconstruction is their distinctive flipper shape: a tight, oar-like profile which hugs the contours of the fin skeletons. However, both muscle studies and soft-tissue data indicate that their limb morphology was quite different to the underlying osteology, and our 'oar-like' depictions are problematic.

Firstly, reconstructions of plesiosaur forelimb musculature show that they were likely powerfully muscled around the shoulders, especially ventrally. Reconstructions of plesiosaur forelimb musculature have been around for almost 100 years and several alternative ideas on the exact configuration are available. They vary from sparingly muscled reconstructions where those massive, plate-like pectoral elements are left mostly free of muscle anchorage (e.g. Carpenter et al. 2010), to models where the entire girdle is swathed in huge muscle attachment sites (Araújo and Correia 2015). The latter seems to reflect the most phylogenetically-informed hypothesis (using data from lizards, crocs and turtles, which seems sensible given on-going uncertainty about plesiosaur ancestry) and - from a purely intuitive perspective - an extensively muscled limb girdle seems more likely than a lightly muscled one. Why develop those huge coracoids if they aren't going to anchor anything?

If the more extensive models of pectoral musculature are correct, we need to consider how the proximal regions of plesiosaur forelimbs would have looked like in life. One key consequence is that, once we link all the pectoral muscles to their insertions on the limb and body, the 'shaft' of the 'oar-shaped' flipper disappears: muscles running along the anterior and posterior region of the humerus fill the pinched, concave regions so that the proximal region is almost as thick as the bony paddle. Much of the proximal humerus becomes buried in muscle dorsally and ventrally too, to the extent that we might imagine the shoulder region was quite bulky in life.

Summary diagrams of plesiosaur pectoral musculature based on Araújo and Correia (2015), with some of my own input on the body outlines (middle and right). Left shows a schematic plesiosaur skeleton (based on Rhomaleosaurus) and a 'traditional' soft-tissue outline, traced from Araújo and Correia (2015). Middle shows the superficial dorsal pectoral musculature predicted by their study - note that it embiggens the pinched proximal region of the flipper by bulking out the anterior and posterior humeral regions. Right shows how data from plesiosaur soft-tissues - see below - changes the flipper shape even further.

In this respect their limb anatomy might look more similar to that of modern tetrapod swimmers – such as whales, seals and turtles – than we typically reconstruct it. We might draw particular comparison to pinnipeds, where a noticeable bulge can be seen at the junction between the forelimb and the torso. The size of plesiosaur pelvic girdles probably indicate a similar muscular condition for the hindlimb and we might assume that they weren't slender-necked, 'oar-shaped' fins either.

Holotype specimen of Seeleysaurus guilelmiimperatoris. Note soft-tissue outlines behind the right forelimb and tail. If you'd like to see these tissues in person, you're too late - the body outlines of this specimen were painted over years ago. Bummer. From Dames (1895).

But these are not the only tissues which distort the outline of the flippers. Fossils of plesiosaur body outlines are very rare, but three specimens (the holotypes of Seeleyosaurus, Hydrorion and Mauriciosaurus - seeDames 1895, von Huene 1923 and Frey et al. 2017) preserve soft-tissues that considerably augment their flipper shape. All three show deep wedges of soft-tissues tapering along the back of the fin skeleton to the flipper tip, with Mauriciosaurus showing tissues - though their shape isn't entirely clear - also present behind the proximal limb regions. There is sufficient consistency across these specimens to suggest expanded paddle tissues were common, and maybe even widespread, in plesiosaurs and, for artists, augmenting our plesiosaur flipper skeletons with these trailing edge tissues should be our standard approach to their restoration.

Hydrorion brachypterygius and its soft-tissue forelimb impressions (the dark, grainy textures behind the fins). From von Huene (1923).

Moving on, artists might also want to note that ideas about highly restricted motion of plesiosaur flippers are being revised. Traditionally, authors such as Carpenter et al. (2010) have argued for limited motion at both the shoulder and hip limb joints, resulting in what I like to call the 'sinking rowing boat' pose: depictions of plesiosaurs with limbs projecting just a little off the horizontal, regardless of what they're up to. Restricted fore- and aft motion seems likely given the elongate shape of limb girdle joints, but whether the vertical movement of the limbs was restricted to tight arcs - perhaps as shallow as a 54° total range - is being challenged (e.g. Liu et al. 2015). Plesiosaur limb girdles were evidently highly cartilaginous in life and estimating their joint motion challenging - most of the information we desire to determine some sense of joint mobility is long gone. But if we assume they had more than the slimmest covering of cartilage in the girdle limb joints - which seems sensible, given the huge size of the girdle joints and their poor match for the limb bone shape - we can assume wide arcs of motion to both limb sets before disarticulation. The exact range of movement remains an open question - unpublished studies hint at even greater motion than other 'wide arc' research, such as Liu et al. (2015) (thanks to Darren Naish for advance word on this) - but artists should not feel confined to the 'rowing boat' pose that we've seen plesiosaurs depicted in for decades. With my artist hat on, I find this very welcome news. Plesiosaurs with limbs perpetually stuck out sideways can look a little static even in the hands of great artists, and their limited poseability has not made them the most interesting subjects to reconstruct. Wider arcs of motion allow plesiosaurs to be depicted in more complex and dynamic poses, and to convey a greater range of behaviours - pirouetting around corners with dipped fins, beating their flippers to attain high speeds, dropping their limbs because they're being lazy... all sorts of stuff. Well done, science, you've made at least one artist a happy person.

Aspects of the neck

My experience with the mass-economising, lightweight long necks of terrestrial or volant tetrapods means the extensively developed vertebrae of longer necked plesiosaurs are of great personal interest. Freed of the constraints of mass reduction, their numerous neck vertebrae are short, highly developed elements with long, robust processes - the exact opposite of the long, simplified structures I'm used to dealing with. Assuming plesiosaur necks were constructed like those of other amniotes (below), they likely anchored powerful muscles along their lengths. In particular, their neural spines are very tall and we can assume they bore enhanced musculature associated with lifting and turning the neck - useful features for long necked animals living in a dense fluid medium. Myological reconstructions suggest that the axial column would bear muscles connecting to the pectoral girdle, producing a deep set of tissues at the neck-torso junction (Araújo and Correia 2015, see pectoral myology diagram above). Artists should equip these animals with chunky, powerful 'reptilian' necks rather than svelte, bird-like variants. I do wonder if thick muscles along the neck might have impacted their neck mobility somewhat - another reason to assume long-necked plesiosaurs were only capable of bending their necks into simple curves (e.g. Zammit et al. 2008).

Amniote neck muscle groups and functionality, modelled by the American alligator Alligator mississippiensis. If the same basic rules apply to plesiosaurs, we should expect many species to have huge muscles and very powerful necks. Diagram concept and muscle layout after Snively and Russell (2007).

The neck/skull articulation of plesiosaurs is also of interest. In many taxa, including Plesiosaurus itself, the posterior face of the skull is displaced anteriorly to the jaw joints. This condition is not unique to plesiosaurs, also being found in some other reptiles including living crocodylians. This 'staggering' of the posterior skull margins might minimise any obvious topographic demarcation between head and neck tissues (the head/neck junction is less obvious in crocodylians than it is in many birds and mammals, for instance) as as well as complicate motion at the head-neck joint. The anteriormost cervical vertebrae and their articulation with the skull would be buried by bone laterally and throat tissues (including muscles and hyoid cartilages) ventrally, and we have to wonder if this envelope of material would limit how far the skull could pivot on the neck. The analogous condition in modern crocodylians seems to bear out this prediction, so perhaps we should not be restoring plesioaurs with mammal- or bird-like cocked heads.

Trunk shape - cross section and lateral profile

Plesiosaurs are often restored with a generic, 'barrel-shaped’ trunks. This is appropriate for some taxa, but not all. It must be said that plesiosaur torso shape is an area of on-going research. I recently spoke with a number of plesiosaur experts on this matter and found aspects like rib and gastralia articulation, the vertical position of the pectoral girdle and so on were somewhat contentious (thanks to Richard Forrest, Aubrey Roberts and Mark Evans for their thoughts). The crux of the issue is that, unlike some reptiles (such as birds or pterosaurs), plesiosaur torso skeletons don't slot neatly together in a single, incontrovertible manner, as is evident to anyone who's seen more than one plesiosaur mount in a museum. Understanding their torsos requires precise appreciation of their vertebral rib articulations, knowing their rib and gastralia curvature in three dimensions, and the benefit of fully articulated fossils for reference. This is quite a list of requirements, and one that is only currently met by a fraction of plesiosaur taxa.

Despite this, detailed reconstruction attempts provide reason to think not all plesiosaurs had tubby, barrel-shaped torsos. Close inspection of vertebral rib articulations and the shape of three-dimensionally preserved plesiosaur torso skeletons allowed O’Keefe et al. (2011) to reconstruct some cryptoclidids with tall, barrel-shaped bodies, and others with dorsoventrally compressed ones (below). In some genera, like Tatanectes, this is augmented further by almost flat dorsal ribs. It is difficult to gauge torso cross sectional shapes from just looking at a typical, half-prepared and flattened plesiosaur fossil, but artists should be mindful that not all species will have circular torso sections. Given how important torso shapes are to a reconstruction, we should check research literature carefully to make the most informed call we can on this aspect of restoring their life appearance.

Cryptoclidid torsos in cross section, with (over conservative) soft-tissue outlines. Modified from O'Keefe et al. (2011).

It is not only the cross section of plesiosaur trunks which are of artistic interest. Neural spine height is not always consistent along the dorsal column, with genera like Attenborosaurus having much taller vertebrae towards the anterior end of the torso. I don't think we know much about the torso cross section of this animal yet, but its vertebral proportions alone imply a proportionally deep shoulder region and a ‘tear-drop’ profile in lateral aspect. This may have been translated into soft-tissue depth in life: deep neural spines over the shoulder might betray a well developed m. latissimus dorsi, a forelimb elevator muscle that could be beneficially augmented for a swimming animal. Interestingly, Attenborosaurus has larger forelimbs than hindlimbs, and it's not entirely daft to wonder if its big shoulder vertebrae and their possible role in beefing out the shoulder muscles reflect forelimb-dominated swimming (see Liu et al. 2015). That's a discussion for another day, of course: the take home for artists here is to pay attention to those trunk vertebrae, and think about how they might influence the long-axis trunk symmetry.

Attenborosaurus conybeari, Jurassic equivalent of those top-heavy gym users who forget about working their legs.

Last month I posted a complaint about poor scientist-led palaeoart - those artworks of extinct animals produced under direct control of scientists to promote research, without any interference from TV companies or book publishers, and yet still end up being objectively flawed at a scientific level. I focused a lot of my criticisms at scientists themselves, as they have final authority over factual aspects of palaeoartworks. That doesn't necessarily clear artists from all blame, but it's naive to think that every artist tackling palaeoart has specific training in palaeontological matters, unrestricted access to technical literature, or the anatomical knowledge required to restore fossil animals without instruction. A scientist's main role in a palaeoart collaboration is bringing rigour and information to the table and, when science-led pieces are objectively poor, we have to wonder what happened to those guiding hands.

Our instinct might be to assume that lack of scientific rigour reflects flippancy towards palaeoart and its impact, and I think that is true - in at least some cases. But in a comment posted after my article, Matt Bonnan proposed that scientifically poor artwork might reflect scientists struggling with their role in the palaeoart process - that is, not really knowing how to instruct an artist, or where the line between scientific and artistic considerations lies. I can believe this is true, too. Lots of people - including many scientists and artists - view science and art as incompatible concepts, and are unsure how to approach projects blending the two. For some folks the idea of contributing to an art project is pretty terrifying, perhaps because they're afraid of being seen as naive, or of their contribution letting a project down.

Whatever the reason, I want to follow my previous criticism with something more constructive: some pointers for how scientists might approach their role in producing palaeoartworks. The take-home message is that scientists concerned about getting paint on their fingers shouldn't worry about the artistic aspects of paleaoartworks. The primary role of a scientist is not to understand colour hues, choice of media, composition and so on, but to comment on objective, factual components of the work. Is the restored species the right size in relation to other species and the environment? Is its head the right shape? Has it been restored with the right soft-tissue anatomy? Answering these and other questions do not require artistic training, or any special training at all for that matter, just application of knowledge that most palaeontologists will already have.

This discussion mainly considers scientists involved in producing a novel palaeoartwork, but will also apply to those reviewing artwork before publication or exhibition. This includes peer review of papers with palaeoartworks, and I encourage editors to make sure these aspects are checked alongside other parts of a paper. After all, if an artwork is being presented as scientifically-credible enough to be included in a peer-reviewed publication, it should be held to the same standards as the rest of the publication. I don't want to beat a dead horse by bringing this up again, but we should recall that palaeoart components can remain in use long after the context of their original genesis has been consigned to history, and artworks associated with papers can be especially prone to long-term use. We should take all opportunities to steer depictions of the past in the most credible directions, hopefully influencing subsequent generations of artists in the best way possible.

So, what should scientists look to critique in artworks, and how might they go about assessing credibility? A perquisite of guiding palaeoart processes is having a concept of what the subject species looked like. This might seem like a patronising comment, with experts replying 'of course I know what [subject species] looks like!', but really think about it - do you really know what the proportions of your subject are, and what they look like when reconstructed against one another? As in, to the point where you could render a reasonable stick-figure version of it? Could you describe what it looks like outside of plain lateral view, and are these interpretations based on modern technical data, not treatments of the same subject by previous artists? Palaeoart is highly varied in scientific credibility and even those works produced by masters of palaeoart may have dated or contain errors. Ergo, palaeoart consultants should form their basic concepts of appearance from images of fossils, tables of measurements and other primary resources, not previous artistic interpretations. This need for caution applies to skeletal reconstructions too, as these become dated and require modernisation as much as any other reconstructions of prehistoric life. So before dusting off that copy of Romer's Vertebrate Palaeontology for another round of consultancy work, or digging out a skeletal from a century ago, consider how kind the last few decades of research have been to those familiar images. Anyone needing an example of how a well-known, seemingly 'safe' skeletal can become dated should check out Scott Hartman's new Dimetrodon skeletal. The animal we all 'know' as Dimetrodon is really Romer's 1927 skeletal - 90 years on, it's looking pretty different.

Role 1 of a palaeoart consultant: know what the basic anatomy of a subject looks like when pinned together. A great poster child for this requirement are pterosaurs, familar animals but with very unfamiliar proportions. It's continually clear from scientist-led pterosaur palaeoartworks that their proportions remain unclear even to specialists, a fact made especially obvious by the continued depiction of large torsos in many species. Spend an afternoon piecing together pterosaur fossils, or even just measurements of their bones, and their tiny bodies - shown here via Quetzalcoatlus sp. - become inescapable.

Once good contemporary and credible references have been amassed to account for the technical side of a project, they can be used as core reference material for all parties on the project, giving a common goal to work towards. A useful shopping list for 'core references' might be a skeletal reconstruction of the subject species (either drawings or a good museum mount; a closely related species might do if the specific skeletal is unavailable, and especially if the subject is poorly known or only subtly different from available reference material), lists of bone measurements and ratios, and literature providing a detailed insight into the anatomy of the subject. Good photos of referred specimens, from as many angles as possible, are always helpful too.

Is palaeoart a reliable source of information about the appearance of a fossil animal? Sometimes yes, but oftentimes, no. Talk slide from my 2014 TetZooCon presentation on the cultural evolution of azhdarchids pterosaurs, showing some of the earlier, zanier attempts to restore these animals. There's so much incredulous anatomy here that artists and scientists should steer well clear of these as reference material and go back to primary sources - fossils, descriptions, measurements - to form the foundation of their artwork. (Psst - TetZooCon is happening again soon, details here)

With these references in hand, regular checks on developing artwork can begin. A rule of thumb in palaeoart is that aspects of an artwork should be justifiable one way or another ("these proportions are because of that, this pose seems OK because of this, this speculation reflects this..."). If they aren't, or the defence for that aspect is suspect, the artwork should be modified until a superior interpretation is presented. We can go a long way to bringing palaeoart credibility up to speed by appraising the following, fact-based elements:

• Anything to do with basic measurements, including the size of the subject relative to its environment and other species, or the proportions of its body. Obtaining metrics from 2D art can be difficult if a subject is obliquely posed or foreshortened, but their rough proportions can be estimated based on their relationship to other body parts. If in doubt, it’s better to get the artist to check their work than to ignore it. Pay particular attention to the proportions of the head to the rest of the body, the size of the torso, and the ratios of the limb bones, as these are prone to errors.
• Whether the skeleton of the subject fits within the restored soft-tissue volumes. Especially notice the shape of the head and teeth, the cross-section and length of the torso, and the bulk of the appendages, as these are often problem areas. Also make sure the position of the shoulders is correct – it is often more challenging to reconstruct the pectoral region than the pelvic, so the forelimb attachment region can end up in strange places.
• Whether the chosen pose breaches predictions of joint articulation. Over-stretched limbs, as well as exaggerated neck and tail poses, are key to look at here.
• Whether appropriate fossil soft-tissues have been factored into the painting. This includes tissue types (e.g. correct integument) and aspects of tissue bulk. Where tissue types are unknown, check that the predicted substitute is based on sensible use of phylogenetic bracketing and comparative anatomy.
• Finally, note whether the species depicted in the artwork were actually contemporaneous, and that the restored environments and climates are appropriate. 

If, via aid of flux capacitored DeLorean, I was consulting for my own azhdarchid art from 2008, I could make lots of suggestions for improvement on purely scientific grounds. The comments here - concerning proportions, limb bone orientations, bone shapes and so on - could be made from any scientist familiar with recent work and interpretations of pterosaur anatomy, and do not require any forays into the artistic side of palaeoart.

Note that none of these aspects stray into areas of artistry, except - sometimes - a need to interpret 3D shapes in 2D art. Moreover, virtually all of these elements relate to commonly studied aspects of fossil forms. All we're doing is taking the same bone shapes and proportions that inform taxonomic or systematic studies, or the ratios and metrics that underlie functional analyses, and applying them to a different project. We're using information that most scientists already know or have immediately to hand, just set to a different tune.

Because of this, palaeoart consultancy is not as arduous a task as it first appears, nor a total time sink. I'm not going to pretend that good palaeoart consultancy is a job you can do in seconds but, once you have basic references established, most comments simply pertain to nudging the reconstruction in the right direction. As with many academic projects, advising on palaeoart requires the most time investment up front, and then relatively little after. Needless to say, the more prepared you are at the start, the less time investment is needed down the line.

And these points - basic as they might seem - will see just about any palaeontologist able to guide and shape palaeoart production. It should be stressed how continued checking along these lines can make an amazing difference to a palaeoartwork, and thus its success at capturing a hypothesis and future legacy. Correcting a scientific goof not only makes a picture more credible, but it often marks a division between a picture being artistically lacking and coming together. There's a reason artists of living creatures (including humans) are so obsessed with the anatomy of their subjects, and that's because it's essential to producing good artwork. Palaeoart is no different, so don't be shy: help your artist get the information and understanding they need to make your science look great.

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A female Pteranodon tries to explain the new Silverstone et al. (2017) paper on Pteranodon taxonomy to the Cretaceous shark Squalicorax. Unfortunately for her, the sharks quite liked the 'Dawndraco' hypothesis.

Last year I posted a couple of overviews of the better parts of the pterosaur palaeoecological record, discussing what we know was eaten by Rhamphorhynchus and azhdarchid pterosaurs, as well as what species ate them. These reviews were tied to a peer-reviewed paper on the same subject which, at the end of Febuary 2017, was published as part of an upcoming collection of pterosaur papers (Witton 2017). This collection, edited by David Hone, myself, and David Martill, is the proceedings of the Flugsaurier 2015 pterosaur meeting and will, when finished, contain over a dozen new insights into pterosaur research, with an emphasis on their palaeobiology. You can check out the existing content here - keep an eye on that site, as there are more papers to come.

With my paper now out (though sadly not open access, but I will eventually be able to post an unformatted version online next year) I thought it would be a good time to take a holistic look at direct fossil evidence of pterosaur lifestyles. What are some of the most interesting examples of pterosaurs interacting with other species? Which purported interactions stand up to scrutiny, and which ones are a little tenuous? And what do they tell us about the all important Big Picture of pterosaur palaeobiology?

Yes, some pterosaurs may well have been seabird mimics

A number of pterosaur specimens have been reported as being associated with the remains of their last meals. Several of these have been lost, found to be erroneously interpreted, or are simply too poorly preserved to interpret their gut content. However, examples of the Jurassic non-pterodactyloids Rhamphorhynchus and Scaphognathus, the Triassic Eudimorphodon, and the famous Cretaceous taxon Pteranodon show reliable insights into their dietary preferences (below). These are virtually all remains of aquatic animals - mostly fish - preserved in intimate association with pterosaur skeletons, either between their jaws, aligned with their throats or within the torso skeleton. One example of a coprolite is known, though it's difficult to say exactly what it contains.

Pterosaurs and their last meals (shaded grey). A, torso of Eudimorphodon; B-D, various Rhamphorhynchus with gut content and coprolite (C), E, Scaphognathus; F, Ludodactylus; and G, Pteranodon. From Witton (2017).

Many of these specimens have been known for several decades, and their evidence of aquatic feeding probably played some part in the stereotyping of pterosaurs as seabird analogues (e.g. Wellnhofer 1991). Nowadays, we need to be a little more circumspect about what they tell us. Yes, they do show that some pterosaurs ate fish and other pelagic prey and, along with results from detailed studies into functional morphology, they help portray certain pterosaur species in the 'classic' seabird niche. Rhamphorhynchus, Scaphognathus, Eudimorphodon and Pteranodon have at least some adaptations consistent with foraging for pelagic prey, such as long wings ideal for marine soaring, 'fish-grab' jaws and adaptations for launching from aquatic settings, as well as occurrences in coastal or marine settings. It would be a little odd if these aquatic-adapted species weren't catching aquatic animals from time to time.

But we can't maintain the older view that these specimens, on their own, undermine the increasingly diverse and nuanced takes on pterosaur palaeoecology hinted at by form-function studies, biomechanics, and modern understandings of pterosaur habitats. We have thousands of pterosaur specimens in museums around the world, of which gut content is known from less than a dozen examples, and in four species. That's not even enough to demonstrate the full dietary range of the species in question, let alone tell us about the ecology of all pterosaurs. Indeed, the scarcity of pterosaur gut content agrees with some new predictions of pterosaur lifestyles in that non-aquatic food sources now suggested for pterosaurs - insects, wormy things, fruits, small tetrapods - have limited preservation potential, particularly outside of Lagerstätten. When factored against common agents of taphonomy and preservation, these hypotheses predict empty bellies in many pterosaur fossils, which is what we find virtually all of the time. It is, of course, difficult to be certain of anything concerning negative evidence, but it's nevertheless useful to note this predicted match between modern ideas and fossil data.

A selection of pterosaur foraging traces - beak tip impressions and scrape marks - from Jurassic and Cretaceous sites. The black-filled elements are the feeding traces, dark grey are manus prints, and light grey are footprints. From Witton (2017).

Evidence that not all pterosaurs were obtaining their food out to sea comes in the form of feeding traces - small, paired impressions and scratch marks created by beak tips (above). These were likely formed by pterosaurs wandering over water margins in pursuit of invertebrates and other small prey, much like extant shorebirds and waders. Indeed, if you walk across a mudflat on a falling tide you can find near identical traces made by living avians mimicking this pterosaur strategy. Somewhat frustratingly, the identities of the pterosaurs that made these tracks remain mysterious. That said, in my new paper, I have - finally - formalised a case for a Late Cretaceous Mexican set of tracks and possible feeding traces (panel D, above) having an azhdarchid trace maker.

Pterosaur feeding evidence: the 'close, but no biscuit' specimens

Inferring palaeoecological details from fossils can be tricky, and it is unsurprising that some purported insights into pterosaur diets and lifestyles are contentious. One of these is the famous and perhaps darkly comic circumstances surrounding the holotype skull and mandible of Ludodactylus sibbicki, a Cretaceous, likely fish-eating Brazilian ornithocheirid found with a sharp, pointed leaf between its lower jaw rami (panel F in the image above). Much of the 2003 description of this specimen (Frey et al. 2003) discusses this association and concludes that ingestion of these plant remains led to the death of the pterosaur. According to this story, the pterosaur accidentally scooped up the leaf, having mistaking it for its usual prey, stabbed the plant material on its throat tissues, frayed the end of the leaf trying to work it loose, but starved to death before it could dislodge it.

I must admit a little scepticism about this scenario. This is not because animals getting things stuck in their mouths is implausible, but because the story presented by Frey and colleagues is pretty presumptive. It infers a lot about pterosaur behaviour, foraging strategies, throat tissue strength and so on that we can't confirm at present. Moreover, the hyoid apparatus - the skeletal support for much of the throat and tongue tissue - is preserved lying on top of the leaf, despite the suggestion that the plant matter was deeply imbedded in the throat tissues. How did that work itself loose with the leaf fatally stabbed between the jaws? The answer to that question - as with a lot of questions about this association - would easily fall into speculation and special pleading about all manner of unknown quantities, and thus has little value to understanding fossil animal palaeobiology. Boring and po-faced as it is, I don't think the unusual Ludodactylus holotype provides enough information to tell us much about pterosaur behaviour, or how this unlikely fossil association came to be.

A similar observation might be made about insect specimens - a dragonfly and lacewing - from the Jurassic Solnhofen Limestone that have torn wings, allegedly from a pterosaur attack (Tischlinger 2000). The logic goes that these otherwise perfectly preserved insects cannot have been attacked by aquatic predators, or else they would have been eaten after their wings were damaged. Failed attack from an airborne predator that would not pursue the injured insects into water is suggested as more likely. Solnhofen deposits do hold pterosaurs that were almost certainly aerial insect hawkers - such as Anurognathus (below, see Bennett 2007 and Witton 2013) - and these might be ideal perpetrators in this scenario.

Anurognathus ammoni was an insect-hawking pterosaur that lived over the Solnhofen lagoon. Has it left feeding traces on fossil insect wings after a failed attack?

 As with Ludodactylus, this set of circumstances is quite elaborate to base purely on damaged insect wings. The extent of their wing damage is considerably greater than we might expect under general 'wear and tear' and foul play was probably involved, but whether it was a pterosaur, a conspecific, or even those disregarded aquatic predators is difficult to say. I appreciate the logic that aquatic predators would eat disabled insects after a failed strike, but animals are not predictable, logic-driven machines: they make mistakes, strike at things they have no intention of eating, get bored, distracted and so on. In all, other than the fact that these insects were almost certainly attacked by something, it might be difficult to say anything more substantial about their final moments.

Pterosaurs vs. dinosaurs, crocodyliforms and... the revenge of the fish

The fossil record gives us an insight on the question "did pterosaurs taste good?", and that answer seems to be "yes". Bite marks, embedded teeth and vomited pterosaur remains indicate that dinosaurs, crocodyliforms and fish all ate pterosaur flesh, at least on occasion (below). Among the more impressive examples of these interactions is a spinosaurid tooth, likely from the Brazilian spinosaurine Irritatorchallengeri, embedded in the cervical vertebra of an ornithocheirid (Buffetaut et al. 2004). Alas, no other evidence of their interaction was evident on the specimen (a series of pterosaur vertebrae) and it's not possible to ascertain much about circumstances that brought these species together.

Evidence of many, many things that ate pterosaurs. A, ornithocheirid cervical vertebrae with embedded spinosaurid tooth; B, azhdarchid tibia with tooth gouges and embedded dromaeosaur tooth; C, ornithocheiroid wing metacarpal with unidentified puncture marks; D, Quetzalcoatlus sp. skull with puncture marks; E, Eurazhdarcho langendorfensis cervical vertebrae with crocodyliform puncture marks; F, Pteranodon sp. cervical vertebra with intimately associated Cretoxyrhina mantelli tooth; G, Velociraptor mongoliensis torso with possible azhdarchid pterosaur gut content; H, probable fish gut regurgitate including Rhamphorhynchus bones; I, associated Rhamphorhynchus muensteri and Aspidorhynchus acutirostris skeletons. Images drawn and borrowed from many sources - see Witton 2017 for details.

The fossil record's most common purveyors of pterosaur murder, however, are not dinosaurs or crocodyliforms, but fish. Apparently out for revenge after learning of all that fishy pterosaur gut content, we've got evidence of fish eating and spitting out pterosaurs, of pterosaurs getting entangled with piscine predators, and even fish bite marks on pterosaur bones. A lot of these pertain to specimens of Rhamphorhynchus and you can read more about them in this post - some of the specimens are exceptional and there's lots to say about them. One of the more famous examples of piscine-pterosaur consumption -  an Italian, Triassic pellet composed of alleged pterosaur bones (Dalla Vecchia et al. 1989) - has recently been reappraised. It's now more reliably interpreted as vomit ball made of bones from the tanystropheid Langobardisaurus (Holgado et al. 2015).

Lesser known, but pretty darned awesome examples of fishes eating pterosaurs are Pteranodon specimens that found themselves at the wrong end of Cretaceous sharks. Several Pteranodon bones reveal bite marks and even embedded teeth from two genera of sharks, the 2-3 m long 'crow shark' Squalicorax and the larger, 6 m long 'ginsu shark', Cretoxyrhina. The former seems to have eaten Pteranodon flesh on several occasions, while evidence of the latter is only currently known from a tooth closely associated with a cervical vertebra (panel F, above). Further work on the latter specimen is currently underway.

 Feeding traces from these sharks are common in Western Interior Seaway fossils and those of Squalicorax are particularly abundant and taxonomically indiscriminate. Given that even giant marine reptiles are among the species consumed by this mid-sized shark, it's often assumed that this animal was a scavenger, biting into whatever free meat floated about America's continental sea. However, it is less certain that Pteranodon was scavenged by Squalicorax, as even a 2 m long specimen would vastly outweigh the largest Pteranodon. It is not inconceivable that an unwary Pteranodon could be grabbed and killed by a stealthy Squalicorax, though I stress this scenario is no better supported than the shark simply chancing acrossa Pteranodon carcass. Whatever the scenario, it's somewhat grounding to think of a weird extinct creature like a pterosaur being devoured by a fairly conventional-looking shark. It's a reminder, perhaps, that Mesozoic life was not a pantomime of exotic, giant reptiles and weirdo evolutionary experiments, and that much of our modern ecosystem was in place many millions of years ago.

The big picture

Looking at the pterosaur palaeoecological record holistically, what patterns emerge? If we look at where the record focuses phylogenetically (below), it's obvious that our records are significantly biased towards certain taxa - Pteranodon, Rhamphorhynchus, and azhdarchids. Even their close relatives, with similar anatomy and adaptations, preservational conditions and so on, don't get much of a look in. There's a few data points scattered here and there, but tumbleweeds run though the palaeoecological data stores for the majority of the group.

Attempting to make sense of the pterosaur palaeoecological record in a holistic way mainly shows how paltry this record remains. It's improved a lot in recent years, but we await evidence of diet and consumer-consumed relationships in virtually all major pterosaur clades. The images at the bottom of this figure are takes on known examples of pterosaur ecology: Rhamphorhynchus ingesting fish, and azhdarchids being devoured by dromaeosaurs. From Witton (2017).

We wouldn't be scientists if we didn't ask ourselves why this is. I don't think it's simply a sampling issue. The pterosaur record is not great, but we are talking about several thousand specimens now - enough that we might start looking at what we don't have as well as what we do. So why does Rhamphorhynchus show 10 palaeoecologically-relevant fossils, but other Solnhofen species only preserve one confirmed piece of gut content? Why do azhdarchids, which are never found in sites of exception preservation and are generally only known from bits and pieces, have a better record than those lineages which are abundant, represented by dozens of complete skeletons, and often found in sites of exceptional preservation? Interestingly, there's no obvious correlation between factors like abundance, preservation quality and palaeoecological data. Several lineages - the ctenochasmatoids (wading pterodactyloids), the rhamphorhynchids (excluding Rhamphorhynchus) and ornithocheiroids (excluding Pteranodon) - have everything going for them in terms of abundant fossils, occurrences in sites of exceptional preservation, and yet they turn up very little in the way of gut content, or evidence of being consumed by other Mesozoic animals.

My take on all this is that there must other factors at play here. We don't get evidence of pterosaur palaeoecology just by throwing more fossils, or better quality fossils, into the mix. I'm sure these factors have some role, but perhaps only in concert with special traits of certain pterosaur groups - maybe behaviours and anatomies - that allow them to have good records. We might have a good record of azhdarchids being consumed by dinosaurs and crocs, for instance, because their bones are often quite big and allow predators to bite them without destroying them. Perhaps we have good palaeoecological insights for Rhamphorhynchus and Pternanodon because of their habits and behaviour - both have strong aquatic adaptations (see this blog postfor ideas on that), and there is a bias towards preservation of aquatic animals in the fossil record. Perhaps this aids preservation of not only palaeoecological data, but also explains why these taxa are our most abundant pterosaurs (>100 Rhamphorhynchus fossils are known, >1000 Pteranodon).

The pterosaur palaeoecological record, then, is perhaps in a transformative state. Though vastly improved over its condition a few decades ago, it requires further augmentation to provide us with significant insights into pterosaur lifestyles, and to explain its biased nature. However, we should not be too pessimistic about the insight it offers into pterosaur palaeobiology: it still provides useful datapoints that can shape our interpretation of flying reptile ecology for several species. Cliched as it is, the take-home message of this project is that any palaeoecologically-relevant pterosaur fossils are worth putting on record. We still have a lot to learn about how these animals lived and behaved, and direct insights are the most reliable ways to do that.

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References

• Bennett, S. C. (2007). A second specimen of the pterosaur Anurognathus ammoni. Paläontologische Zeitschrift, 81(4), 376-398.
• Buffetaut, E., Martill, D., & Escuillié, F. (2004). Pterosaurs as part of a spinosaur diet. Nature, 430(6995), 33-33.
• Dalla Vecchia, F. M., Muscio, G., & Wild, R. (1989). Pterosaur remains in a gastric pellet from the Upper Triassic (Norian) of Rio Seazza valley (Udine, Italy). Gortania, 10(1988), 121-132.
• Frey, E., Martill, D. M., & Buchy, M. C. (2003). A new crested ornithocheirid from the Lower Cretaceous of northeastern Brazil and the unusual death of an unusual pterosaur. Geological Society, London, Special Publications, 217(1), 55-63.
• Holgado, B., Dalla Vecchia, F. M., Fortuny, J., Bernardini, F., & Tuniz, C. (2015). A reappraisal of the purported gastric pellet with pterosaurian bones from the Upper Triassic of Italy. PloS one, 10(11), e0141275.
• Martin-Silverstone, E., Glasier, J. R. N., Acorn, J. H., Mohr, S. & Currie, P. J. (2017). Reassesment of Dawndraco kanzai Kellner, 2010 and reassignment of the type specimen to Pteranodon sternbergi Harksen, 1966. Vertebrate Anatomy Morphology Palaeontology, 3, 47–59.
• Tischlinger, H. (2001). Bemerkungen zur Insekten-Taphonomie der Solnhofen Plattenkalke. Archaeopteryx, 19, 29-44.
• Wellnhofer, P. (1991). The illustrated encyclopedia of pterosaurs. Salamander Books.
• Witton, M. P. (2013). Pterosaurs: natural history, evolution, anatomy. Princeton University Press.
• Witton, M. P. (2017). Pterosaurs in Mesozoic food webs: a review of fossil evidence. Geological Society, London, Special Publications, 455, SP455-3.

Benjamin Waterhouse Hawkin's (1858?) sketch of amphibious marine reptiles, including a large shambling ichthyosaur. Image borrowed from Frank T. Zumbach's Mysterious World.

One of the most charming aspects of mid-19th century palaeoart are those amphibious marine reptiles: depictions of ichthyosaurs and plesiosaurs that hauled themselves onto rocks or beaches to rest, or lunge with open jaws at passers by (above). To modern eyes these images look naive and quaint, a clear reminder of how far our understanding of fossil animals has progressed in the last two centuries.

Of course, art has a habit of imitating life and, a good 150 years after amphibious marine reptiles became unfashionable in palaeoartworks, Ryosuke Motani and colleagues (2014) published a new marine reptile suggested to be capable of locomotion on land as well as in water: the ichthyosauriform Cartorhynchus lenticarpus. This Chinese, Early Triassic species is anatomically remarkable in several respects. Although reminiscent of early ichthyosaurs in overall shape, it has a considerably reduced snout, seems to lack teeth, is just 20 cm from snout to vent despite indications of osteological maturity, and bears enormously long forelimbs. Though unique when first discovered, another, much larger Cartorhynchus-like species has since been found in the same deposits, Sclerocormus parviceps. Together, these animals form a clade at the base of Ichthyosauriformes known as Nasorostra, the 'nose beaks', referring to a defining feature where their nasal bones reach the jaw tip (Jiang et al. 2016).

Holotype specimen of Cartorhynchus lenticarpus. Note the enormous forelimbs with their expansive unossified wrists, indicated by the distal phalanges being well posteriorly displaced from the upper arm bones. From Motani et al. (2014).

The amphibious habits of Cartorhynchus are primarily based on its unusually large forelimbs and small body size, it being reasoned that Cartorhynchus could drag or propel itself over exposed sediments like a mudskipper, turtle or pinniped. I find this idea fascinating: an ichthyosauriform that was at home outside of water? Cartorhynchus certainly deviates from ichthyosaur anatomy and evolutionary trends enough to inspire inquiry about its weird bauplan - if it was not amphibious, it might be doing something else equally unexpected. The amphibious Cartorhynchus hypothesis has received surprisingly little detailed attention online, save for coverage of a 2014 press release and this excellent primer article at Tetrapod Zoology, so there's scope for a closer look at this idea. What is the evidence for amphibious habits in Cartorhynchus, and how does this concept fit models of early ichthyosaur evolution?

The functional basis for an amphibious lifestyle in Cartorhynchus

Motani et al. (2014) present a fairly detailed argument in favour of amphibious habits in Cartorhynchus. The chief lines of evidence are those expansive forelimbs, but it's not just their size that matters: their enormous, unossified carpal regions are also significant. Several early ichthyosauriforms have poorly ossified carpal bones but the unossfied region in Cartorhynchus flippers is proportionally bigger by some margin. This would allow these ordinarily-rigid marine reptile flippers an unusual degree of flexibility and optimise them for terrestrial locomotion. Flipper-based terrestrial motion is surprisingly tricky because its users tend to be suboptimally designed for movement out of water and they almost always have to overcome drag forces acting on the body as well as shove themselves around. Moreover, substrates associated with coasts and waterways tend to be unstable, yielding under pressure and being challenging for even proficient terrestrial animals. These factors mean flippers can easily dig into substrate or slip across it rather than propel their owners about, and it's easy to see why beaching is fatal for so many specialised aquatic species.

Studies (using robot turtles!) suggest that rigid flippers are generally poor at terrestrial locomotion and may even be incapable of moving animals over some surfaces (Mazouchova et al. 2013). A bendy flipper, in contrast, works well, allowing the forelimb to flex before the substrate moves, spreading the weight of the animal over the distal limb and allowing the proximal flipper region to elevate and support the body (Mazouchova et al. 2013; Motani et al. 2014). The unusually expanded flexion zone in Cartorhynchus forelimbs would be well suited to this purpose, and certainly much better at this task than those of other ichthyosaurs. We might note, as an aside, that the lack of flexion zones in other marine reptile flippers, such as those of plesiosaurs, might be good reason to doubt their ability to crawl over land.


The downside of having lots of cartilage in a long flipper is that they are weaker against bending than a more ossified one, so their utility as a walking limb lessen as the forces involved in moving the body increase. It's here where the small size of Cartorhynchus comes into play. Small size equates to low body masses and smaller forces associated with lifting the body, less structural demand on the flipper, and reduced drag effects from the sliding belly. As is so often the case in evolution, small body size might be an enabler for evolutionary experimentation in Cartorhynchus, allowing it to perform feats that its bigger relatives just couldn't even if they were also equipped with giant, bendy fins.

The tail of Cartorhynchus is incompletely known but it's anatomical and phylogenetic proximity to the completely-known Sclerocormus suggests that its tail was long, flexible, and lacked any sort of fin or fluke (Jiang et al. 2016). A relatively simple tail lessens the risk of it dredging sediment or catching on debris during terrestrial locomotion and its flexibility might have permitted its use as a prop or even propulsive organ: fish such as the Pacific leaping blenny show how a long, bendy tail can be used to powerful effects in semi-terrestrial locomotion (Heish 2010, also below). Combinations of fin and axial motion in land-crawling fish can be surprisingly effective over a range of substrates (Standen et al. 2016) and we might assume similar options were available to Cartorhynchus.
Putting all the strands of the amphibious Cartorhynchus hypothesis together, I don't see reason for excessive suspicion about the idea of beach hauling nasorostrans. At the core of the pro-amphibious argument is that Cartorhynchus (and perhaps, by extension, Sclerocormus) has weird anatomy that requires an explanation - it's just too different from other ichthyosauromorphs to pretend it wasn't doing something unusual, maybe even unexpected. Amphibious behaviours are an explanation that seem to chime well with provisional form-function investigations and seem a sensible hypothesis at this time. That said, we should be appropriately cautious in our interpretations of these animals: our understanding of nasorostrans is in its infancy and alternative, currently-unexplored functional hypotheses could explain their anatomy as well, or better, than the amphibious concept in future. Fingers crossed that these animals receive more dedicated functional investgiations in years to come.

Or maybe more robot turtles. Either is good with me.

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References

• Carroll, R. L., & Zhi-Ming, D. (1991). Hupehsuchus, an enigmatic aquatic reptile from the Triassic of China, and the problem of establishing relationships. Philosophical Transactions of the Royal Society of London B: Biological Sciences, 331(1260), 131-153.
• Hsieh, S. T. T. (2010). A locomotor innovation enables water-land transition in a marine fish. PloS one, 5(6), e11197.
• Jiang, D. Y., Motani, R., Huang, J. D., Tintori, A., Hu, Y. C., Rieppel, O., ... & Zhang, R. (2016). A large aberrant stem ichthyosauriform indicating early rise and demise of ichthyosauromorphs in the wake of the end-Permian extinction. Scientific reports, 6, 26372.
• Mazouchova, N., Umbanhowar, P. B., & Goldman, D. I. (2013). Flipper-driven terrestrial locomotion of a sea turtle-inspired robot. Bioinspiration & biomimetics, 8(2), 026007.
• Motani, R., Jiang, D. Y., Chen, G. B., Tintori, A., Rieppel, O., Ji, C., & Huang, J. D. (2015). A basal ichthyosauriform with a short snout from the Lower Triassic of China. Nature, 517(7535), 485-488.
• Prondvai, E., Stein, K., Ősi, A., & Sander, M. P. (2012). Life history of Rhamphorhynchus inferred from bone histology and the diversity of pterosaurian growth strategies. PLoS One, 7(2), e31392.
• Standen, E. M., Du, T. Y., Laroche, P., & Larsson, H. C. (2016). Locomotor flexibility of Polypterus senegalus across various aquatic and terrestrial substrates. Zoology, 119(5), 447-454.
• Tibblin, P., Berggren, H., Nordahl, O., Larsson, P., & Forsman, A. (2016). Causes and consequences of intra-specific variation in vertebral number. Scientific reports, 6, 26372.

Abelisaurid Kryptops palaios recieves some TLC in Early Cretaceous Niger, while duck-faced Anatosuchus minor sneaks out of the water. But what's with the swollen, bulging look to the abelisaur's face? (Concept by Chidumebi Browne. Award yourself an extra biscuit if you spot the homage to one of my palaeoart heroes in this scene.)

Reworked version of my 2012 Tyrannosaurus painting, now in it's third guise. There's something about this painting which recalls reconstructions from 1906 rather than those of 2016.

The skeletal anatomy of Tyrannosaurus rex is probably better known and studied than the skeletons of many living animals, but its soft-tissues - and thus much about its life appearance - are poorly represented by fossil remains. Thus, virtually all of our ideas about muscle bulk, soft-tissue body shape and integument have to be reconstructed by phylogenetic proxy and functional prediction. As with all dinosaurs, we've historically felt pretty confident that Tyrannosaurus was entirely scaly, but relatively recent discoveries of filamented tyrannosauroids in China (Xu et al. 2004, 2012), as well as a growing mountain of fuzzy coelurosaur fossils, point to a different conclusion: that Tyrannosaurus was adorned in simple filaments - hair-like equivalents of feathers. Skin impressions for more derived tyrant species - the tyrannosaurids - have proven rare in fossil record (Hone 2016) and, though rumours have circulated about some, they have largely escaped formal description and publication. In the absence of better evidence, the most parsimonious modern takes on everyone's favourite tyrant have involved a fuzzy covering.

In the recent months two papers have challenged this idea. The first, by Thomas Carr and colleagues (2017), purports to find osteological correlates of scales on the facial anatomy of the tyrannosaurid Daspleteosaurus, which they argue (along with other lines of evidence), to suggest crocodylian-like facial tissues and sensitivity. The second, by Phil Bell et al. (2017), describes scaly skin impressions from multiple postcranial regions of a Tyrannosaurus skeleton, and argues that the distribution of these impressions implies a uniform (or near uniform) covering of scales across the body, without much in the way of fuzz.

Because this is Tyrannosaurus, media sites and bloggers have spilled great amounts of ink over these stories. The scientific press has often been far from objective or unbiased. Popular articles have suggested Jurassic World fans might have 'won' the debate over scientists, that science fans are 'due' a return to scaly tyrants after 'losing' Pluto, and that the findings mean 'all is well in the dinosaur world'. The implication is a ridiculous one, like evidence of scalier tyrants is a moral victory rather than a test of a scientific hypothesis. But while the popular press has been celebrating the new papers, members of the palaeoblogosphere have been less enamoured with the findings. Trey the Explainer suggests that Bell et al.'s work doesn't really change what we already knew about tyrant integument, and thus does not invalidate many existing reconstructions. Andrea Cau posits that interpretations of scaly tyrants reflect our prejudices more than science, and that taphonomic factors may explain the absence of filaments. Brian Switek has concerns that the skin patches are too small and spread too widely to give a complete picture of the integument, and echoes concerns about taphonomic interference. The collective response seems to be a defensive one, protecting concepts of filamented tyrannosaurids from a resurgence of a more traditional, scaly model. Would any other dinosaur get this treatment? Perhaps not: as Brian explains in his recent post, this reaction is the T. rex celebrity effect at full bore.

Supermegafluffy Tyrannosaurus, from 2015. They were simpler times.

I've painted many fluffy Tyrannosaurus in the last few years (above) and quite like the idea of everyone's favourite 6 tonne dinosaur bonecrusher being a giant plush toy. However, we also have to concede that our ideas of Tyrannosaurus skin have been largely informed by prediction, not direct data, and that popular, long-held notions are as ripe for scientific revision as any other (lest we forget other famous examples of this - Brontosaurusand Ornithoscelida). Moreover, although some critics are suggesting the papers don't tell us anything new - rumours of scale impressions have been circulating for years - these recent studies give us the first rigorously documented, peer-reviewed glimpse into Tyrannosaurus skin anatomy. This is new, allowing us to form our own opinions on Tyrannosaurus appearance based on actual data, not hearsay. So, rather than putting our gloves up to defend our prior model, I wonder if we should be exploring how this new data might transform our perception of Tyrannosaurus life appearance. That these new studies present conflicting data to our expectations is not grounds to be upset, annoyed or defensive. To the contrary, they allow us to use real data - not predictions - to refine our ideas of tyrannosaurid appearance and evolution. For those of us interested in dinosaurs as real entities, and not movie monsters, that's a good thing.

What, exactly, has been argued about scaly tyrants?

A lot of the popular write ups of these recent papers include errors and misrepresentation, so let's recap what is actually being argued about Tyrannosaurus skin. A common social media reaction to Bell et al.'s work is that they've presented 'a patch' of skin, and are extrapolating from that. We need to debunk that right away: they've not described a single patch, but multiple small patches from the neck (alas, exactly where on the neck isn't reported), the top of the pelvis, and the base of the tail (below). All the samples stem from the 'Wyrex' specimen (HMNS 2006.1743.01). The most extensively represented area is the tail base, which has the largest single piece of fossil skin - 30 cm². The other skin samples are not as large, some being just a few centimetres across. Each patch shows the same skin type: uniform, tiny 'basement scales', each less than 1 mm across (Take note, artists: you would not see Tyrannosaurus scales until you were being eaten by their owner). Similar scale patches, also described by Bell et al. (2017), have been found on the torso and tail regions of other tyrannosaurid species, implying similarly scaled regions in these taxa.

Tyrannosaurus skin patches from the neck, pelvic area and tail of the 'Wyrex' specimen as illustrated by Bell et al. (2017). The scale bars for the scale imagery are 5 mm (b - e) and 10 mm (f-h). These things are tiny, and we can assume the skin of the animal would look smooth or leathery in life.

These osteological correlates combine with the skin impressions to collectively show Tyrannosaurus as scaly across much of its face, somewhere on its neck, over the pelvic region and along the tail base (below). So far as we can tell, this picture seems consistent with osteological correlates and skin sampling from wider Tyrannosauridae. That's pretty extensive coverage, ruling out the presence of fibres in places that we know other dinosaurs - including other tyrannosauroids - were fuzzy, and implies that tyrannosaurids were mostly scaly. I'm particularly startled at the scales over the hip region as they curb even the long 'fibre capes' we see in some modern tyrant reconstructions, like the famous Saurian Tyrannosaurus. The fact that the scales occur in places known to be ancestrally filamented for tyrants is also intriguing: Bell et al. (2017) speculate that they may be modified feathers - that is, the same as bird scales - rather than a reversion to lizard or croc scales. Hold that thought, we'll come back to it soon.

Everyone's doing maps of Tyrannosaurus with integument details nowadays, and I want in. Note that this is Tyrannosaurus specific, and does not feature scale data from other tryannosaurids.

What's in the gaps?

The million dollar question is what was present between these scaly regions: more scales, or fibres? This is a major point for many respondents to the Carr et al. and Bell et al. papers, as it decides whether we keep our interpretation of Tyrannosaurus as an - at least partly - fuzzy animal. With our scale distribution map as a starting point, several options are available. The first is that fuzz was present in regions not yet represented by skin remains or osteological correlates. This would mostly imply the top of the torso (Bell et al. 2017), but may also be parts of the back of the head, some aspects of the neck (depending on where the neck skin impression came from) and maybe the end of the tail. Over on Twitter, Patrick Murphy has presented a reconstruction which shows what this might look like. I must admit to finding it quite amusing, sort of like T. rex has put on a shawl to visit the opera.

But how dense could these fuzzy patches have been? Bell et al. (2017) suggest that dense fibrous coverings are doubtful, noting that large living mammals avoid patches of thick insulating fibres to aid heat loss. This has not gone down well with some critics, who cite studies of feathers preventing over-heating instead of facilitating it. An oft-cited study in this regard is Dawson and Maloney (2004), who found emu feathers block virtually all solar radiation from the skin, preventing them from overheating in solar exposure that causes similarly-sized hairy mammals to seek shelter.

Feathers: great at blocking solar radiation, also great at trapping body heat. Note how cooking hot these ostriches are on their necks, heads and legs, while the feathers are mostly ambient temperature. This isn't because the body isn't warm, but because the feathers block the heat signature entirely, trapping all that heat around the body. As surface area:volume ratios drop as animals get larger, it stands to reason that the benefits of blocking solar radiation give way to a need shed heat. Image from Wikipedia user Arno / Coen, CC BY-SA 3.0.

Feathers, however, are not magic structures that defy fundamental physical laws of insulation, nor do they liberate animals from the challenges of heat loss at reducing surface area:volume ratios. Beyond a certain size, shedding excess body heat is difficult for any terrestrial animal, and it gets tougher as they get larger. King and Farner (1961, p. 249) described feathers as having "an extremely high insulating value to the feathered surfaces" and a rich literature of studies on modern birds shows that feathers are as effective at trapping body heat as they are blocking solar rays (e.g. King and Farner 1961; Kahl 1963; Philips and Sandborn 1994; Dove et al. 2007). We can almost see them as a little too effective, leading many birds to develop heat-dumping adaptations to circumvent their own insulation, such as highly vascularised, non-feathery body parts as well as a repertoire of postures and behaviours (maximising exposure of unfeathered body parts; flapping wings; urinating on their legs) that aid cooling (e.g. Kahl 1963; Arad et al. 1989; Philips and Sandborn 1994). So yes, feathers are terrific at protecting birds from environmental heat, but that limits their ability to release metabolic heat from their own bodies.

If living birds find feathers a little warm, despite their relatively high surface area to volume ratios, we have to assume a theropod weighing anywhere between 6-14 tonnes is going to find big areas of dense filaments a challenge to thermoregulation too. It is not unreasonable to assume blankets of fibres could be a problem for big tyrants. The counterargument here is that Yutyrannus huali, a largish tyrannosauroid, does have dense fibres everywhere. But Yutyrannnus seems more lithe than Tyrannosaurus - perhaps just 10-25% of its mass, depending on the estimates (Bell et al. 2017) - and lived in a more vegetated, and thus shadier, habitat (Bell et al. 2017). A neat comparison Bell et al. (2017) make along this line uses living rhinos, where hairier species live in shadier settings than the virtually naked ones. In light of this, the reduction of filamented regions, and perhaps lessening their density, is a reasonable inference for animals of the size and habitat of Tyrannosaurus, and would reflect thermoregulatory responses to scaling and shade availability seen in living animals.

Large tyrannosauroids, like Yutyrannus huali, show that dinosaurs weighing perhaps 1.5 tonnes could be covered in feathers. But does this reflect the fact that this animal lived in shadier, vegetated habitats than the tyrannosaurids? This idea isn't silly: adaptation to specific circumstances has a major role to play in shaping animal skin anatomy, and could well explain why some tyrants are fuzzy, and others seem less so. (If you want to see the rest of this picture, check out this Patreon post)

Could Tyrannosaurus have had extremely fine, widely-distributed filaments - perhaps similar to something like elephant hair? This isn't entirely falsified by the new data, although the skin impressions we have show no evidence of such a covering despite preserving tiny integument details. Granted, animal filaments can be extremely fine, and they might be beyond the preservation potential and mechanics of even these high-res impressions. However, if we're arguing for filaments of this size and patchiness then - certainly for artistic purposes - we should concede that the animal would be essentially scaly, in the same way that most rhinos, elephants and hippos are essentially naked (below). From a thermoregulatory perspective, short, sparse filaments could make sense as these have the surprising ability to draw heat from the body in modern elephants, helping them stay cool (Myhrvold et al. 2012). Given the potential for overheating under dense filament coats in giant animals (Bell et al. 2017), I see this as more plausible than a 'cloak' of fibres between our scaly waypoints.

Scaly, minimally-filamented Tyrannosaurus. There's some tufts on the neck, but that's it. Is this model more consistent with the thermoregulatory requirements of a 6-14 tonne animal?

A last interpretation of this new data is that Tyrannosaurus was actually just scaly, with no fibres whatsoever. This is the most contested suggestion made by Bell et al. (2017), but it's not unreasonable with our current knowledge. Existing skin data, representing seven parts of the body if you pool all the distinct skull correlates and postcranial points (add several more if you want to extrapolate scale patches from other tyrants), shows enough scales and consistency in the scalation pattern that uniform scale coverage is not a ridiculous or indefensible concept. I appreciate that some folks will point to regional fuzziness of animals like Kulindadromeus in response, and its sharply defined areas of different integument types, and that's valid point. But we can also point to plenty of dinosaurs with extensive or entirely scaly hides and - if there's any value to linking body size and thermoregulatory regimes - they're a better match to Tyrannosaurus body mass than any known fuzzy species. For the time being, wholly scale models fit our existing data just as reasonably as partly fuzzy ones so, archaic and counter-intuitive as it seems - a scaly Tyrannosaurus is not an unreasonable interpretation for the life appearance of this animal, given our current data.

Beyond Tyrannosaurus: 'unlocking' dinosaur skin constraints

My take-home from these new papers is that our models of Tyrannosaurus skin have not crystallised, but we're a little more constrained in how we can imagine this animal, and have to concede a scalier appearance than many of us thought likely. But the implications of the Bell et al. study go beyond Tyrannosaurus in implying new ways to think about dinosaur skin evolution. With incontrovertibly fuzzy animals lining much of the the tyrannosauroid tree and its root, our scalier Tyrannosaurus gives us one of the best examples of a dinosaur replacing fuzz with scales. This is a far-reaching conclusion for those of us interested in dinosaur life appearance, complicating the already confusing evolutionary pattern of scale and fuzz distribution within the group. Ideas that some dinosaurs could be 'secondarily scaled' are supported by this discovery, and we have to wonder if classically fuzzy lineages - including many other theropod lines - are as tightly locked into fuzz, fibres and feathers as we once thought. Could large dromaeosaurs be a little lighter on fuzz than we imagine? Did Therizinosaurus look less like a giant pigeon and more like a walking Christmas dinner? We don't know, but now have reason to wonder.

Fluffy Tyrannosaurus juveniles, one of the possibilities created by the idea that tyrannosaurs might have avian-like 'dynamic' skin. The recovery of scales in non-scaly clades is not as simple as it might first appear!

Furthermore, the notion that Tyrannosaurus scales could be modified feathers (Bell et al. 2017) opens possibilities about mixes of filaments and scales. It's important to realise that not all scales are alike: 'reptile'' scales' are developmentally and genetically distinct from those we see in birds, which are actually secondarily modified feathers (Chang et al. 2000; Dhouailly 2009). Reptilian skin cannot be forced to grow feathers or filaments (Chang et al. 2000) and is developmentally static: once scales are formed, they're with them for life. Bird skin, however, is far more dynamic, and allows for all manner of ontogenetic and even seasonal variation in scale:feather ratios, changes to feather types, and modification of scale size (Lennerstedt 1975; Stettenheim 2000). If, as suspected, our tyrannosaurid skin samples represent fibrous integument masquerading as a scaly one, is this a sign of a bird-like 'unlocked' skin configuration where epidermal dynamism was possible? If so, Tyannosaurus could have changed appearance considerably with age (fluffy when small, scaly when big - above) or season (reflecting changes in climate or behaviour)? It must be stressed that we don't have any direct insight into these sorts of changes at the moment, and the hypothesis of tyrannosaurid scales being modified feathers needs testing. But the irony - we might have data indicating Tyrannosaurus could change its appearance readily, vindicating debaters on both sides of the scaly and fuzzy debate - is not lost on me. Maybe, just this once, everyone wins?

Summing up time

Let's tie this all together. A lot of ambiguity remains about the skin of Tyrannosaurus and its relatives, and it's not wise to hold any opinion about their life appearance too strongly at present. However, unduly downplaying the creep of scaly evidence into the tyrannosaurid fossil record isn't useful or logical. The skull skin correlates and fossil skin patches show that scales were present in numerous, widely-distributed parts of the body, and - until we see evidence to the contrary - this is good reason to assume scalier Tyrannosaurus than we might be used to. And yes, this does mean that some of our favourite, fluffier interpretations are now directly contradicted by fossil data, and consigned to our ever growing book of historic, discredited reconstructions. But this is always a possibility in palaeontology: our views of these animals are only ever hypotheses based on a sparse, biased fossil record, and every new discovery risks overturning someone's favourite concept. The fact we're able to move on from these reconstructions is positive, as it means we're a little less uncertain about the past, and a little closer to the truth.

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References

Palaeoart has never been a particularly diverse artform. Since the early 1800s most palaeoartists have pursued art attempting to depict fossil animals in realistic ways, with stylistic variation mostly along the spectrum of how obvious our brush strokes and pencil lines are, and how much detail we add. In recent decades we've seen artists deepening their dedication to realism with hyperrealist palaeoart, artworks which look like they've been snapped by high-speed cameras with crisp focuses and ultra-high levels of detail.

But not all palaeoartists are taking this approach. Some take a step away from not only high levels of detail but also realism, producing palaeoart with a more stylised and even abstract bent. Though few in number, the growing roster of ‘stylised’ palaeoartists represent an exciting new frontier for palaeoart. In varying artworks along spectra other than tidiness and detailing, these artists are producing unconventional works recalling pop art, classic western animations, heraldic crests, perspectiveless Medieval art and more. Among the most fascinating aspects of these works is their capacity to maintain respect for scientific credibility even when producing stylised, non-realist art. The forms may be simple or sharply angular, the colours may be garish, but we can still tell what the subjects are, what they are doing, and get a sense of their anatomy.

...which brings us to Johan Egerkrans's Alla tiders dinosaurier. If you like stylised palaeoart, you should check out this book.

Swedish artist Johan Egerkrans is part of this emerging group of unconventional palaeoartists. Emerging onto the online palaeoart scene only recently, his work has already generated a fanbase and widespread acclaim. It's easy to see the appeal of his creations. Distinctively angular, full of personality and recalling great works of American animation, his digital artworks emphasise and almost caricature the form of fossil animals without undue distortion of their form or disregarding fossil data. Attention to details, anatomy and colours make his work interesting to look at despite it's simplicity compared to traditional modern palaeoart. We're not just seeing generic cartoons of fossil animals, but highly stylised versions of contemporary, scientifically credible palaeoart, informed by a clear appreciation for modern wildlife and the natural world. Notice the pupil colour change between his adult and juvenile Microraptor (below), variable integuments on Gorgosaurus (above), fine attention to animal poses and behaviour, and so on. His use of traditional compositions and poses prevent his work becoming overbearing: in this regard, his work is less intrusive, and even perhaps less cartoony, than some artists employing ‘realistic’ animals in hyper-dynamic poses and compositions.

Egerkrans' parent and offspring Microraptor. Look past the stylisation and this is a pretty accurate take on Microraptor anatomy, right down to the iridescent black plumage. Note the pin feathers and dark pupil on the juvenile - very sensible speculations for juvenile maniraptorans. © Johan Egerkrans.

Each Egerkrans work radiates personality: his animals have real character, and it’s almost impossible not to imagine them taking part in animated vignettes. Several of his works have a strong sense of mischief and dark humour, another rarity among palaeoartworks. I’m particularly tickled by his scene of a capybara running away from terror bird Titanis (below): the bird has a mania that captures real birds at their most frantic and chaotic, while the drab mammal looks overwrought, panicked, but also like it’s going to write a strongly worded letter to the Daily Mail about all this. Comparisons of Egerkrans’ creations to stylised fossil animals rendered for the big screen are inevitable, and mostly leave us wondering what the heck everyone else is doing wrong. Hollywood, give this man a job!

Titanis and capybara star in Hilarious Scene of Violence. Capybara won an Oscar for its eyebrows. © Johan Egerkrans.

Johan was kind enough to send me a copy of his recent book, Alla tiders dinosaurier, which I thoroughly recommend you check out. There’s no English translation at the moment (one might happen at some point) but the artwork speaks volumes alone and the design and print quality is excellent - it's a nice book to have, even if you're unable to read the text. The follow up, Flygödlor och havsmonster, which focuses on marine reptiles and pterosaurs, is due out later this year. Both are published by B Wahlstroms, and can be purchased from Bokus and other Swedish book retailers (sorry, American readers, there are complications around shipping these books to the USA at the moment). You can check out the art of both books on Facebook, Artstation and Johan's blog. If you're Stockholm-based, you can also check out a dinosaur exhibition featuring the Alla tiders dinosaurier work, which is running until the end of September.

Earlier this month I asked Johan if he’d like to chat to me about his art, books and palaeoart philosophy, and he’s taken time out of his schedule to give the following interview. With thanks to him for taking time to respond to my questions, it’s time for me to stop gushing about his work and hand you over to the man himself…

Europasaurus holgeri - twice. These portraits are of the same animal using the same specimen and the same view, but one is restored with extreme shrink-wrapping (above) and the other has a more generous amount of facial tissue (below). But which one is more plausible, and can we even tell from fossil bones alone?

You can't move around palaeoart circles on the internet nowadays without someone being criticised for 'shrink-wrapping' their reconstruction. This refers to the convention of restoring extinct animals with minimised soft-tissues, allowing details of muscle layouts and major skeletal contours to be seen in allegedly healthy living animals. At its most extreme, this includes clearly visible ribs and vertebrae, tissues sunk into skull openings, ultra-prominent limb girdles and skinny, sinewy legs. We owe the term 'shrink-wrapping to sauropod expert and SV:POW author Mathew Wedel who, in a 2010 article, compared the contour-hugging soft-tissues of these restorations to items wrapped in tight plastic for transport.

Shrink-wrapping is a well known convention among those interested in palaeoart but is a relatively modern invention. Palaeoartists restored ancient animals with relatively bulky soft-tissues until the end of the 20th century to an extent where visible deep-tissue anatomy is genuinely exceptional in pre-modern palaeoart (a well known exception are ichthyosaur sclerotic rings, reflecting erroneous interpretation of these structures among early palaeontologists - see Buckland 1836). Shrink-wrapping became popular as conservative reconstruction approaches became dominant in the 1970s and went on to become a standard palaeoart convention soon after. Many, perhaps most, of the restorations produced by late 20th century artists employed shrink-wrapping and it remains conspicuous in artwork produced today. It has even spawned related traditions, such as tightly cropping fur and feathers to ensure animal shapes remain obvious, and has influenced approaches to restoring colour and skin texture, these elements being used to outline the topography of underlying bones. Famous shrink-wrappers include artists like Gregory S. Paul and Mark Hallett, who tend to be on the less dramatic side of the tradition, showing slight contours of the skull features alongside lean, though well-muscled, bodies and limbs. More extreme shrink-wrappers, like Ely Kish and William Stout, have works where shrink-wrapping is taken to a wholly unrealistic level. Gaping vacuities exist between neck vertebrae; rib cages and limb girdles bulge from the torsos; limbs are extremely thin and faces are lipless and gaunt. It’s difficult not to look at some of these works and not think of starving animals or even decaying remains: they do not look like healthy, virile beings.

William Stout's Quetzalcoatlus, posted at Love in the Time of Chasmosaurs, has to be the most shrink-wrapped being ever rendered in paleoart. If it had any less tissue we'd be looking at moulds of the internal organs.

We might assign three reasons for the popularity of shrink-wrapping. The first is that its development coincided with a reinvention of dinosaurs as bird-like, active and powerful animals rather than oversized, under-muscled cold-blooded creatures. The athletic appearance of shrink-wrapped dinosaurs chimed with this renaissance and contrasted newer art from the plodding, perhaps over-voluminous animals of previous generations. Shrink-wrapping is not a dinosaur-exclusive tradition of course, but the popularity of these reptiles means that palaeoart conventions applied to dinosaurs are inevitably followed in artworks of other species. Secondly, images of prehistoric animals as heroically-built, powerful beings are preferred by many merchandisers and palaeoart fans, these interpretations most closely matching the erroneous but popular portrayal of prehistory as a savage struggle for survival, where only the most powerful animals survived. Thirdly, shrink-wrapping allows palaeoartists to ‘show our work’, demonstrating that the anatomy underlying the skin of a restored animal matches the osteological information provided by fossils.

How shrink-wrapping became unfashionable

Nowadays, shrink-wrapping is losing popularity among some parties as scientists and artists note a simple, but obvious problem: modern animals are generally not shrink-wrapped in the way we draw their extinct relatives. The most famous counter-shrink-wrapping arguments are in All Yesterdays (Conway et al. 2012) but something of an anti-shrink-wrapping movement was underway from the mid-2000s onward. Some now argue that, while champions of the rigorous reconstruction movement were right to draw attention to the true shapes of fossil animals and to emphasise their form in art, they might have gone too far in thinning out skin, muscle, fats and other tissues. Few animals have deeply sunken tissues over skull fenestra or distinctions in skin colour and texture correlating with skeletal anatomy, and no animals witnessed outside of veterinary clinics have detailed limb bone outlines projecting through their skin. Even reptiles - meant to be the living poster boys of shrink-wrapping - have a suite of elaborate, contour-altering soft-tissues. They include voluminous fat deposits; large amounts of wrinkly, saggy skin; eyes which bulge prominently from their sockets; deep lip tissues which fully sheath their teeth; jaw muscles which completely fill and swell from their skull housing; thick or pointed scales and, in some species, even expansive, mostly cartilaginous noses.

Matt Wedel's touching plea to end shrink-wrapping, from 2011. The struggle is still real: if you have spare paint, pixels clay or graphite, please donate generously.

Nowadays, many view skeletal elements as providing an important palaeoartistic foundation for soft-tissue shape, but concede that overlying tissues must have smoothed-over skeletal contours to produce 'softer' body forms. Indeed, there's something of an collective interest in knowing how deep extra-skeletal tissues can get. The answer, it seems, is 'very'. The necks of many birds and mammals are often flexed at much higher angles than we would assume based on their external appearance because their overlying tissues are so thick that the entire neck skeleton posture is hidden (Taylor et al. 2009). The muscles and bones of major anatomical elements – such as necks and proximal limb segments – can also be obscured under skin, fat and integument. Contour-altering structures like horns, spikes, spines, combs, humps, armour, fins, and webbing are often composed of soft-tissue, and the large, savage-looking teeth of mammals and lizards can be completely obscured by facial tissues. We need only look at x-rays of living animal species to see their often-startling lack of correlation between external appearance and internal anatomy.

Even seals get in on this action, as evidenced from this Irish Seal Sancutary x-ray. Their site appears to be down at time of writing, but SV:POW! has this image hosted there for the time being.

It's from this general train of thought that a  push for more bulk, fuzz and fat in palaeoart has been born, and this general philosophy is lining up well with fossil data. We have direct evidence that the bodies of ichthyosaurus (Stenopterygius) and mosasaurs (Prognathodon) bore tall fins and paddle extensions that vastly exceeded the limits of their skeletal margins (McGowan and Motani 2003; Lindgren et al. 2013). Preserved body outlines of ichthyosaurs and plesiosaurs show deep tissues which created smooth, streamlined torsos that are much bulkier than the underlying skeleton (Frey et al. 2017). Fossils of early horned dinosaurs (Psittacosaurus), Tanystropheus and ‘mummified’ hadrosaurs (multiple taxa) show extensive muscle volume that bury their skeletons as well as elaborate structures – soft-tissue filaments, combs and skin membranes – that defy ‘shrink-wrapping’ conventions (e.g. Mayr et al. 2002; Renesto 2005; Bell 2014). The feather outlines on innumerable fossil theropods show that they were just as densely feathered as modern avians, and the fuzzy ‘halos’ of fossil mammals and pterosaurs suggest they too were also adorned with deep layers of filaments. Several pterosaur fossils (Pterodactylus, Pterorhynchus) also preserve unexpectedly broad neck tissue outlines which contrast against their thin, tubular neck vertebrae, as well as elaborations of crest tissues that create body outlines more voluminous than those predicted from musculoskeletal restorations (e.g. Frey and Martill 1998; Czerkas and Ji 2002). The 'shrink-wrapping hypothesis' is being falsified with regularity.

Select fossilised body outlines of exinct taxa: no shrink-wrapping here. A, plesiosaur Mauriciosaurus fernandezi, B, ichythyosaur Stenopterygius quadriscissus; C, dromaeosaur Sinornithosaurus millenii. A, after Frey et al. 2017; B after McGowan and Motani 2003.

Anti-anti-shrink-wrapping

But while cries of 'bulkier, deeper, fuzzier!' are generally well-placed in palaeoart discussions, we should be careful not to overshoot the mark. Amid the cry for deeper tissues, we might be overlooking the fact that some living creatures are somewhat shrink-wrapped - at least in some regions. In fact, virtually animals have areas where their extra-skeletal tissues are shallow and skeletal contours are visible. Common areas of thin tissue include the ends of limbs and tails; the midline of the sternal region; and some areas of the face, such as the frontal and nasal regions; the ‘cheek region’ (over the jugal in birds and reptiles, and the zygomatic arch in mammals), and the lower margins of the bottom jaw. Our own anatomy is no exception to these trends, as is borne out by the extremely well-studied tissue depths of human faces (e.g. Stephan and Simpson 2008) or the simple act of looking in a mirror. The osteoderms of sauropsids are another example of close interaction between skin and bone: as with modern armoured reptiles, extinct scaly sauropsids with extensive osteoderm arrangements probably looked pretty darn like their fossil remains - in other words, kinda shrink-wrapped.

There is no tissue, only Zuul.

In reality, there is a spectrum of tissue depth in living species and some are more 'shrink-wrapped' than others. While no healthy living animal attains the most extreme levels of shrink-wrappery portrayed in palaeoartworks, certain lizards, fish, and crocodylians have anatomies which are more shrink-wrapped than average, possessing large areas of relatively thin, skull-hugging tissues which recall shrink-wrapped art. These thin tissues are highly characteristic of these species and are something something palaeoartists would want to capture if restoring these animals from fossils. We would miss this, however, if we assume that all animals have their tissue volume settings cranked up to maximum.

These observations mean we have to be careful with applying a general philosophy to shrink-wrapping rather than scientific investigation. Tissue depth is evidently not a matter of palaeoartistic style or fashion, but a biological variable we should be aiming to predict and infer. If we're aiming to approach this topic like scientists, we should look to see what fossils and comparative anatomy can tell us about tissue depth to make informed, specific predictions about extinct animal appearance and avoiding a one-size-fits-all 'anti-shrink-wrap' philosophy. So, is there anything in the fossil record that elucidates how deeply buried animal skeletons were under muscle, skin and so on?

Looking for clues of 'shrink-wrapped' tissues

Frustratingly, one of the first lines of evidence we have to jettison are those body outline fossils. As great as they are, they can be of limited use for determining subtle variation in tissue thickness as their shapes are readily altered by taphonomy, preservation styles and even our own preparation work. Regions of thin tissue depth will be were especially sensitive to destructive processes and are easily obliterated by imperfect preservation or human error, so their chances of preservation are minimal. Phylogenetic bracketing is also of limited utility because the vastly different cranial architecture of extant and extinct animals makes such investigations almost meaningless. Non-avian dinosaurs, for instance, have skulls which are neither truly croc-like or bird-like, and it's probably not sensible to assume their extant relatives provide reliable insights into their facial tissues.

Predicting regions of thin tissue is thus largely left to comparative anatomy - predicting minimised tissue volumes using fossil bones and the living structural analogues. Among extant species, we see shrink-wrapping largely applying to animal faces, so if we investigate the skulls of ‘soft-faced’ animals like mammals, monitor lizards, snakes, and certain birds, and compare them to species with shrink-wrapped faces, like turtles, crocodylians, chameleons and well-ossified fish, we might find characteristics that correlate with facial tissue depth. These will then give us some criteria to assess tissue depth in fossil species. I've had a go at this, and suggest that osteological attributes related to facial tissue depth include:

How might we predict shrink-wrapping in fossil animals without good soft-tissue remains? It's challenging, but these attributes might give a general idea. From top to bottom: Burchell's zebra (Equus quagga burchellii); water monitor (Varanus salvator); Alligator mississipiensis and Arrau turtle (Podocnemis expansa).

Openness of skull architecture. The skull openings of softer-faced animals - including the temporal muscle openings, orbits and nares – tend to be large. At their most extreme these openings are not fully bordered by bone (e.g. many mammal orbits and nares, the lower temporal fenestrae of lizards). Larger skull openings necessitate a larger fraction of face structure be composed of soft-tissue, such as muscle, organs, and cartilage, and this overwhelms the contours of the bony skeleton to make a 'soft-faced' species. The nasal cartilages of monitors and mammals, as well as bulging mammalian jaw muscles, are examples of this. Conversely, shrink-wrapped species have smaller cranial openings, which impose physical limitations on how much soft-tissue can form the shape of the face. Muscles and organs might protrude from these somewhat, but their impact on facial structure is less than that of species with large skull openings, and more of the face shape reflects bony contours

Rugosity. Soft-faced animals tend to have smooth bone textures with limited or no areas of rugosity, whereas the skulls of shrink-wrapped species have large areas of rugose textures, often corresponding to specific epidermal features (e.g. scales or keratinous sheaths - see below and Hieronymus et al. 2009). This factor largely seems to reflect the proximity of epidermal tissue, which can leave characteristic textures in species with tightly-bound skin. Soft-faced species generally lack this rugosity because muscles, fat and voluminous integuments (fur and feathers) don’t leave broad osteological features (Hieronymus et al. 2009), or simply because their skin is displaced far enough from the bone that it doesn't alter its surface. We might also note that the skull contours of soft-faced species are generally more rounded than those of shrink-wrapped species, which can be crisp and sharp. Rugosity is a particularly useful criterion because it can show the presence of tight skin tissues with some precision. If one part of a skull is rugose, and another isn’t, there’s a good chance that the smoother region had a different tissue configuration which could - among other things - reflect a deeper or 'softer' facial covering.

Fossil skulls - like those of the centrosaurine Centrosaurus apertus - are covered with features that allow us to predict aspects of their facial skin. Often - as is the case here - they suggest fairly low-volume structures, like scales and horn sheaths, which generally don't deviate too much from the underlying bone (yes, I know there are exceptions, but we're looking for major trends here). Centrosaurus skull redrawn from this Wikipedia photo, data on facial tissue correlates from Hieronymus et al. (2009).

Pits, grooves and foramina. Shrink-wrapped species tend to have large numbers of perforations in their skulls, while soft-faced species show the opposite (Morhardt 2009). This is particularly evident around their jaws and presumably reflects the greater capacity for soft-faced animals to carry nutrients and sensory information through their soft-tissues, whereas shrink-wrapped animals are forced to run nervous and vascular networks through their face skeletons.

Correlates for epidermal projections. Elaborate skin projections – such as soft-tissue horns or crests - leave characteristic osteological signatures (Hieronymus et al. 2009). Given that these projections can alter animal faces quite substantially from the underlying skull shape, the presence of these is a clear indication that the species was not shrink-wrapped. We would expect a lack of correlates for epidermal projections in shrink-wrapped species.

As is often the case with zoological topics there are exceptions to these observations that preclude using any one of these criteria in isolation to determine tissue depth (e.g. smooth bone textures can underlie thin naked skin, so are not always a hallmark of deep tissues). However, applied collectively, they might give a general insight into how shrink-wrapped or 'soft-faced' an extinct animal was. I'm encouraged to see that these proposed osteological features of soft- and shrink-wrapped faces covaried in the past as much as they do for modern species. This doesn't mean these criteria are 'correct' as goes their relationship to tissue depth, but at least shows there's variation in their skull architecture that we can recognise as equivalent to that of modern species, and it isn't unreasonable to think the variance might reflect the same anatomical factors.

If we apply these criteria to some fossil taxa, what predictions might we make? The roomy, smooth-boned and foramina-lite skulls of cynodont-grade synapsids and fossil mammals match predictions for ‘softer-faced’ species, and this might be true of some fossil reptiles – like sauropod dinosaurs - too (this is not a new conclusion: both Matt Wedel and Darren Naish have been saying similar things about sauropods for years). If right, the 'soft-faced' sauropod that greeted you at the start of this post might be more likely that the shink-wrapped toilet-headed version we're so familiar with. At the other end of the spectrum, the highly textured, pitted bones and solidly-built skulls of ankylosaurs and anamniotes meet our criteria for shrink-wrapping very well, and they likely had facial anatomy tightly conforming to their skull shapes.

Applying the criteria outlined above might help us roughly sort predict 'shrinkwrapped', 'soft-faced' or intermediary conditions in extinct taxa. The placements of the animals here are only rough, but give an indication of their relation to the tissue-depth criteria outlined above. Fingers crossed that some of these will be corroborated or refuted with soft-tissue discoveries in future.

Careful examination of fossil skulls allows us to also predict partial or regionalised shrink-wrapping in species where some aspects of their facial anatomy conformed to the underlying bone, and others did not. An example of this configuration is demonstrated in some living lizards, like gila monsters, which have skull textures strongly indicating minimal tissue depth over much of their skull but smooth, foramina-lite jaw margins. In life, these animals have shrink-wrapped dorsal skull regions and snouts, but vast, fleshy lips, which is what we might predict based on their skull anatomy.

Partial facial shrink-wrapping seems apt for many fossil species. Gorgonopsians, for instance, might not have soft faces like living mammals as their snouts and foreheads are quite rugose and their nasal openings are small (e.g. Kammerer 2016). These features might indicate the presence of tighter skin over the snout. However, they have few jaw foramina and relatively open regions for jaw musculature, so they might have been fleshier around their jaw margins and at the back of head (below). Tyrant dinosaurs have skulls with relatively small openings compared to some of their theropod relatives, rugose snout textures, several hornlets (Carr et al. 2017), as well as a slightly elevated foramina count (Morhardt 2009). This cranial anatomy is consistent with tighter tissue depth in several areas, if someway short of a fully lipless, crocodylian-like degree of shrink-wrapping. Many pterosaurs show pitting and vascular canals embedded into their jaw margins, and some species have indications of tight sheathing on their crests and jaws, but the presence of striated bony crests – correlates for epidermal projections– as well as large skull openings and smooth bone textures in other parts of the skull, indicate that their faces might not have been entirely skeletal.

Was gorgonopsian Inostrancevia shrink-wrapped or soft-faced? According to the criteria of this post, maybe a little from column A, a little from column B.

Time and testing will tell whether these criteria are a genuinely useful means to predict facial anatomy. I hope - as with other aspects of extinct animal appearance - that genuine research into this issue will be carried out one day. Criteria to predict tissue-depth are a desirable tool for any palaeoartist as it's simply more honest and scientific: if we're serious about this reconstructing extinct animals gig, predictive methods and sound hypotheses are infinitely better than sticking to our personal hunches, guesses or erring on what looks coolest. Regardless of whether we can predict tissue depth or not, the take home here is that we should not approach our artwork having already decided how thin or fat the tissue volumes of our subjects will be. There is probably not a single ‘universal truth’ that can be said about restoring tissue depth for all animals, whether we err toward thicker or thinner: the right tissue depth is the most defensible and best rationalised on for each subject and its constituent body parts.

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References

1.5 Arsinotherium zitteli trotting about Eocene Egypt, looking a bit like they could be advertising farm products. But what's with those more elaborate than usual horns?

The horns of the giant, Egyptian, Oligocene afrotherian Arsinoitherium zitteli are probably a key factor in its status as one of the better known fossil mammals. Though perhaps not quite as popular as mammoths or sabre-toothed cats, this 3 m long, four-horned species has enough osteological charisma to warrant display in many museums as well as starring roles in books and films (including, cinema fans, narrowly missing out on an appearance in the 1933 King Kong). And unlike a fossil rhinocerotid (to which it is not at all related), Arsinoitherium doesn't need us to imagine the shape of its ornament in life: two enormous horns project over the end of the snout and another pair of smaller, sub-vertical horns grew above the eyes.

Recently, I painted a portrait of Arsinoitherium for an upcoming book project and, based on my understanding of epidermal osteological correlates, I threw a keratinous sheath over the entire horn set (below). This is not a typical reconstruction - Arsinoitherium has been reconstructed with 'regular' mammalian skin (perhaps better termed 'villose skin' - Hieronymus et al. 2009) on its horns for decades but, as we all know, popularity and longevity don't always equal 'credibility' when it comes to fossil animal reconstructions.

Arsinoitherium zitteli, sporting antelope-like horn sheaths.

Shortly after this image was shared online, Darren Naish, he of Tetrapod Zoology (and the upcoming TetZooCon meeting, which you should definitely attend if you're in the UK and reading this article), had a question: had I checked horns without keratinous sheaths, like deer antlers or giraffe ossicones? It turns out that these are the more typical artistic analogues for Arsinoitherium horns, and their reconstruction without a keratinous sheath reflects this interpretation. It wasn't a question I could easily answer because I'd zeroed in on a keratinous sheath quickly in my research for the image and, in a major palaeoart faux pas, hadn't given due consideration to other options. Simultaneously, neither of us could argue for any model of Arsinoitherium horn coverage confidently because no-one has looked into this in any detail. There are some ideas in the literature, but they are fleeting and conflicting (keratin sheaths - Anonymous 1903; Andrews 1906; Osborn 1907; or skin, Prothero and Schoch 2002; Rose 2006).

It's difficult to turn away a good palaeobiological mystery, and because I like to make sure my work is as credible as it can be, I followed this question up with more research. I reasoned that the structure, development and surface texture of the three major types of mammalian headgear - horns, ossicones and antlers - could be compared to Arsinoitherium horns to see which, if any, is the best match and indicator of life appearance. Looking into this has been very informative and might be of interest to fellow palaeoartists as well as those interested in cool fossil animals, so I thought I'd share my thoughts and process here. We'll start by looking at Arsinoitherium horns themselves, then move through modern potential analogues, and finally compare them at the end to see which model seems most apt.

Arsinoitherium horns: growth, structure and surface texture

PV M 8463, the most famous of all Arsinoitherium skulls, as illustrated in Andrews (1906). Note the dotted lines across the horns - they mark the end of the preserved skull and the start of reconstructed elements.

As noted above, Arsinoitherium has two pairs of horns: a larger anterior set, which grows out of the nasal bones and over the snout, and a smaller, second pair formed from the frontal bones, above the eyes. Both sets are highly conspicuous and dominate the skull, the weight of the anterior pair presumably accounting for the development of a bony bar between the nostrils in mature animals (Andrews 1906; Court 1992). Note that the Arsinoitherium horns we're used to seeing in museums are partly reconstructed and thus of limited use as reference material. Most exhibited skulls are based on NHMUK specimen PV M 8463 (above), a 'moderately sized' adult specimen (Osborn 1907) in which neither horn is complete (Anonymous 1903; Andrews 1906). This skull was among the earliest Arsinoitherium skulls collected from Egypt but was restored rapidly once it arrived back in London. A 1903 report describes how the skull was:

"...brought home by Dr. Andrews from Egypt, and after cleaning, strengthening, and the restoration of parts deficient on the left side by modelling from the right side, is now exhibited in the central hall of the Natural History Museum in Cromwell Road."

Anonymous, 1903, p. 530

The fact that some parts of the skull were in less than stellar shape is evident from this photo of PV M 8463 (from the NHM's data portal): note the variation in colour and texture, reflecting places of reconstruction against real bone. Thus, while the familiar Arsinoitherium museum skull is a useful reference for morphology, illustrations and descriptions in technical literature will be more informative for reconstructing their integument. I've based my assessment mostly on Charles Andrews (1906) monograph, as well as that of Court (1992).

Structure. Both horn pairs of Arsinoitherium are relatively simple in gross shape and maintain the same basic morphology throughout their lives (below), though the horns of mature animals are wider, taller and more pointed than those of juveniles. The figures presented in Andrews (1906) show an increase in anterior horn base length from 41.6% in the smallest specimen to over 56% in the largest. Both horn sets are hollow, with vast internal cavities being supported by sheets of trabecular bone. In some places the exterior bone walls are surprisingly thin, only 5-10 mm (Andrews 1906).

Arsinoitherium zitteli skull ontogeny. I wonder if the horns of the largest skull should be reconstructed as longer and taller, given their arcs in the completely known skulls and gentler tapering of other nasal horn specimens (e.g. Sanders et al. 2004). Skull drawn from Andrews (1906), skull measurements by me.

Surface texture. The base of the horns are marked by deep, broad and branching neurovascular channels running from the facial region onto the horns themselves. The horn shafts are rugose on account of many deep pits, grooves and branching channels aligned along their long axes (Andrews 1906; Sanders et al. 2004). The horn tips of young animals have an especially spongy texture at the tip, presumably reflecting growth of the horn core (Andrews 1906). These textures are not typical of the rest of the skull, which are of a more typical, smooth mammalian variety even in regions where skin was probably in close proximity to the bone (e.g. the zygomatic arch, over the braincase). This is an important distinction, implying that a different epidermal configuration - different skin types, in other words - was present on the horns compared to the rest of the skull.

Having learned something of the Arsinoitherium condition, let's take a look at how modern horns, antlers and ossicones compare...

Analogue 1. Bovid-style horns (keratinous sheaths over a bone core)

Bovid horns typify a widely used approach to cranial ornamentation and weaponry across Tetrapoda. They are perhaps the simplest approach to producing a sturdy cranial projection, being little more than a bony horn core covered in a hard keratinous sheath and are permanent feature in almost animals that bear them. The one exception is the pronghorn, which sheds its horn sheath annually (it also isn't a bovid). Biology, eh: can't we have one rule without an exception?

Bovid (bighorn sheep, Ovis canadensis) horn anatomy. From Drake et al. 2016.

Structure. Bovid-style horns are composed of a hollow bony core lined with trabeculae that strengthens an otherwise thin-walled structure (Drake et al. 2016). The bone portion only occupies the basal portion of the horn, anchoring ever-growing bands of keratin that grow from the bone-keratin interface, not at the horn tip (below). This means that the tip of the horn sheath is the oldest part of the structure and that the base of the sheath is the youngest. Because keratin sheaths are inert, dead and tough tissue, they cannot be remodelled once they are formed. This dictates that the growing bony core has to forever comply with the shape of the horn sheath and cannot change shape much over time. Size changes can be accommodated as wider and longer sheath layers can cover expanding horn cores, but it is not possible to form a more complex shape - say a branch or spur - at the tip of the horn. And before anyone mentions pronghorns: their horn branches are entirely soft-tissue: the bony core retains a simple shape.

Schematic bovid horn growth, adapted from an illustration in Goss (2012).

Surface texture. Deep, oblique foramina and branching neurovascular canals characterise the surface texture of bovid horn cores. This rugosity profile is most pronounced in younger animals, but is maintained to a lesser extent in adults - in many bovids, the horns never stop growing, they just slow down a great deal. This texture is not unique to horns but accompanies many structures with keratinous sheathing, including claws and beaks (e.g. Heironymus et al. 2009). A sharp lip and particularly deep rugosity can mark the transition from horn to facial skin.

Analogue 2. Giraffe ossicones (skin over ossified dermis)

Giraffes have awesome skulls with two - and often more - ossicones that are covered in the same skin as the rest of their faces (Davis 2011). Their approach to cranial ornamentation seems unique to giraffes and their fossil relatives but might be an apt model for aberrant extinct forms, so is worth reviewing here. Clive Spinage (1968) provides an excellent overview of ossicone structure and development: the following is taken from his work.

Structure. Ossicones are low humps or columnar protuberances, continuous with the surrounding skull anatomy but formed from dermal ossifications, not outgrowths of skull bones. They eventually fuse with the skull in adult life but, unlike the underlying skull bones, ossicones are solid and very dense - they are described as having 'ivory-like' in compactness and hardness by Spinage (1968). Mature specimens show increasingly complex shapes including development of swollen tips on the frontoparietal 'horns', as well as hornlets and bosses across the major 'forehead' ossicone. Having an adaptable, living integument is essential to this process, as the ossicone covering needs to change shape to reflect the changing size and complexity of the underlying bone.

Giraffe skulls are full of sinuses, but they do not extend to their ossicones, which are extremely dense. From Spinage (1968).

Surface texture. Generally smooth with oblique foramina in juveniles and young adults, but increasingly gnarly in mature animals (more so in males). The continued ossification of dermal tissues produces a conspicuous pitting and 'flaky' rugosity profile that overgrows the surrounding skull bones and obscures the textures from earlier growth stages. In mature males, this rugosity can overgrow the entire upper surface of the skull and enhance the height and ornamentation of the ossicones considerably.

Young adult male giraffe skull by Wikimedia user Nikkimaria, CC BY-SA 3.0. Note the flaky, irregular textures of the ossicones and their complex shape: they are much more intricate and developed than those of less mature animals. There's room for more irregularity and texture on this skull, too: the skulls of old males look like they have cathedral spires growing from their faces.

Analogue 3. Deer antlers (bony projections atop cranial pedicles)

The familiarity of deer antlers allows us to forget what remarkable and unusual structures they are. Present almost universally in male deer (and in female reindeer), these elaborate, sometimes enormous structures are cast and regrown each year using a regenerative process that is the source of much anatomical and medical interest - no other mammal can regenerate such a complex appendage in this way, and the speed of the regeneration process is remarkable. Antlers are so unusual that they are only partly useful to our discussion here: we are primarily interested in antlers when they are covered in their velvet (specialised antler skin), as this is most comparable to the likely Arsinoitherium condition. Antler skin itself is interesting as, although it is continuous with the skin of the underlying pedicle, it lacks sweat glands and arrector pili (the tiny muscles that pull hair up or give us goosebumps) (Li and Suttie 2000). The antler pedicle (the permanent bony base) in contrast, is covered in the same type of skin as elsewhere on the body (Li and Suttie 2000).

A happy-looking moose (Alces alces) with his fuzzy antlers. Note the visible blood vessels on the underside of each palm. Photo by AlbertHerring, in public domain.

Structure. Both antlers and pedicles are solid, and antlers can - by virtue of growing at their tips - become more complex as they grow, developing from single spurs into networks of brows, tines and palms. As with giraffes, antler skin needs to be living and adaptable to facilitate this: a covering of inert keratin would preclude this form of growth.

More Alces antlers, this time without velvet. Note the long, branching channels. By Wikimedia user Nkansahrexford, CC-BY-4.0.

Surface texture. Antlers have variably developed rugosities consisting of conspicuous, long and branching channels impressed into smooth bone or around prominences and tubercles. These grooves are the impressions of blood and nervous networks that facilitated rapid antler growth. These textures are easily discerned even from a distance, and thus contrast with the texture of the pedicles, which are smoother and lined with relatively shallow, narrow and long impressions of vascular networks. It is unusual for hairy skin to leave such a significant osteological scar on underlying bone: typically, this form of epidermis leaves little to no remnant on skull bones (Hieronymus et al. 2009).

Arsinoitherium vs the analogues

Having looked at three major types of cranial projection in living animals, which - if any - best match the condition in Arsinoitherium?Giraffe ossicones are incomparable to Arsinoitherium horns in several aspects, perhaps the most significant being their increasing complexity and development of flaking bone textures in later life. Furthermore, the development of giraffe ossicones from bony growths in dermal tissues suggests a fundamentally different relationship between skull and dermis than of Arsinoitherium, where the bony horn component represents skull bones alone. There's enough differences here to question whether giraffe ossicones are a good model for the life appearance of Arsinoitherium horns.

In being formed of polished, deeply vascularised bone, deer antlers are closer approximations of Arsinoitherium horns. However, there is so much weirdness associated with deer antler formation and tissues that they almost remove themselves from meaningful comparison to permanent skull horn cores. The fact that antler velvet, as hairy skin, is (to my knowledge) unique in leaving deep vascular channel impressions is a major issue here, implying that either antler bone is unusually susceptible to neurovascular imprinting (do they grow so fast that they grow around their blood vessels?) or that velvet is better at altering bone textures than other skin types. Both scenarios point to antlers having some endemic oddness about them, which complicates their use as a model for life appearance of non-antlered species.

All is not lost with the cervid data, however: antler pedicles are comparable to Arsinoitherium horns in being permanent outgrowths of bone, and they also have neurovascular impressions. However, these shallow grooves compare poorly to the deeper channels and pitting of Arsinoitherium horns. Indeed, there is little about antler pedicle texture to distinguish them from the surrounding skull bones, whereas the opposite is true for Arsinoitherium.

Our comparisons improve with the bovid horn condition, which seems to chime with the Arsinoitherium skull in many regards. Both are hollow outgrowths of skull bones supported by internal trabeculae; both have bone textures characterised by deep, bifurcating neurovascular channels as well as conspicuous longitudinal grooves and oblique foramina; and both maintain the same basic shape throughout growth - excepting some basic changes in base width and horn length. Further similarities include the development of particularly deep rugosties at the base of the horn cores, which is evident in at least large Arsinoitherium skulls (Andrews 1906). This interpretation is consistent with one of the longer (but still rather short, if we're honest) interpretations of the blood vessel impressions in Arsinoitherium:

"These channels evidently lodged blood-vessels which served for the conveyance of blood to or from the covering of the horn, and judging from the marked way in which both these vessels and those on the anterior face of the horns impress the bone, it seems probable that the covering was hard and of much, the same nature as that clothing the horn-cores of the cavicorn ruminants."

C. Andrews (1906), p. 7

So...

Of the three models looked at here, it seems the basic structure and textural package of bovid-like horns best matches what we see in Arsinoitherium. Moreover, unlike the antler or ossicone models, there's no obvious mismatches with this configuration: pretty much everything we would correlate to a bovid-like horn anatomy seems present on or in the Arsinoitherium skull. The idea that a keratinous sheath might have existed in Arsinoitherium might seem odd, but it is not that outlandish given the apparent ease through which keratinous sheaths evolve. This is, after all, the tissue which has covered just about every claw, hoof, nail, horn, cranial dome and beak that has ever existed, whereas ossicones and antlers seem like specialised, clade-restricted approaches to cranial projections. The functionality of hollow Arsinoitherium horns is further reason to suspect a horn sheath. Studies of bovid horns suggest hollow cores and keratin sheaths compliment each other biomechanically, optimising the horns for for impact dissipation (Drake et al. 2016 and references therein). Stripped of a keratinous sheath, we find that hollow horn cores are great at transmitting energy but are brittle and prone to buckling and fracturing under heavy loading. It's only with a tough, fracture resistant keratin sheath that these structures can avoid breaking under heavy use so, if Arsinoitherium employed its horns for anything vaguely physically demanding, they probably needed a keratinous sheath.

It's possible, of course, that these structures were just for show, but they do look like they had a function beyond display. It occurs to me as I write this that this scene recalls the painting from Ghostbusters II. I guess we'll call this guy Vigo.

Putting all this together, I feel the case for a keratinous sheath over the Arsinoitherium horn sets is reasonable, at least so far as it can be made with publicly available data. Aspects of morphology, growth, surface texture and - perhaps - functionality seem fully consistent with a bovid-like horn configuration, whereas other potential models are less comparable. From an artistic perspective, this is exciting: horn sheaths can be extremely elaborate structures and exaggerate the size of the horn core considerably, so Arsinoitherium might have been far more extravagant in life than we have previously imagined. I've tried to hint at this with my reconstructions - remember, this animal wasn't just a funny-faced rhinoceros!

References

• Andrews, C. W. (1906). A descriptive catalogue of the Tertiary Vertebrata of the Fayum. Publ. Brit. Mus. Nat. Hist. Land. XXXVII.
• Anonymous. (1903). A New Egyptian Mammal (Arsinoitherium) from the Fayûm. (1903). Geological Magazine, 10(12), 529-532.
• Court, N. (1992). The skull of Arsinoitherium (Mammalia, Embrithopoda) and the higher order interrelationships of ungulates. Palaeovertebrata, 22(1), 1-43.
• Davis, E. B., Brakora, K. A., & Lee, A. H. (2011). Evolution of ruminant headgear: a review. Proceedings of the Royal Society of London B: Biological Sciences, 278(1720), 2857-2865.
• Drake, A., Donahue, T. L. H., Stansloski, M., Fox, K., Wheatley, B. B., & Donahue, S. W. (2016). Horn and horn core trabecular bone of bighorn sheep rams absorbs impact energy and reduces brain cavity accelerations during high impact ramming of the skull. Acta biomaterialia, 44, 41-50.
• Goss, R. J. (2012). Deer antlers: regeneration, function and evolution. Academic Press.
• Hieronymus, T. L., Witmer, L. M., Tanke, D. H., & Currie, P. J. (2009). The facial integument of centrosaurine ceratopsids: morphological and histological correlates of novel skin structures. The Anatomical Record, 292(9), 1370-1396.
• Li, C., & Suttie, J. M. (2000). Histological studies of pedicle skin formation and its transformation to antler velvet in red deer (Cervus elaphus). The Anatomical Record, 260(1), 62-71.
• Osborn, H. F. (1907). Hunting the Ancestral Elephant in the Fayûm Desert: Discoveries of the Recent African Expeditions of the American Museum of Natural History. Century Company.
• Prothero, D. R., & Schoch, R. M. (2002). Horns, tusks, and flippers: the evolution of hoofed mammals. JHU press.
• Rose, K. D. (2006). The beginning of the age of mammals. JHU Press.
• Sanders, W. J., Kappelman, J., & Rasmussen, D. T. (2004). New large-bodied mammals from the late Oligocene site of Chilga, Ethiopia. Acta Palaeontologica Polonica, 49(3), 365-392.
• Spinage, C. A. (1968). Horns and other bony structures of the skull of the giraffe, and their functional significance. African Journal of Ecology, 6(1), 53-61.

Thalassodromeus sethi, a juvenile Mirischia asymmetrica, and half a spinosaurid hang out in Cretaceous Brazil. The spinosaurid wants to go home.

One of my favourite pterosaurs is the Brazilian thalassodromid Thalassodromeus sethi: a large (4-5 m wingspan) Cretaceous azhdarchoid known only from a broken skull and cranial fragments of disputed affinity (Kellner and Campos 2002; Veldmeijer et al. 2005; Martill and Naish 2006). Characterised by an especially large bony cranial crest, toothless jaws and a robust skull construction, Thalassodromeus gained fame (and it's name, which translates to 'sea runner') from a presumed habit of skim-feeding. Long-time readers or pterosaur aficionados will know that multiple studies have suggested pterosaurian skim-feeding was unlikely on anatomical grounds (we discussed this most recently here and here) and was especially improbable for large species on account of the huge energy demands of ploughing large, blunt jaws through water (e.g. Humphries et al. 2007). A lack of skim-feeding habits does not make Thalassodromeus any less interesting however: it's a large, charismatic animal with a heavy dose of pterosaur weirdness, so there's still plenty to like. I recently had reason to overhaul the Thalassodromeus painting from my 2013 book (above) and took the opportunity to revisit my understanding of this animal's anatomy. The process had me fall for Thalassodromeus' cresty charms all over again, and I've taken this as impetus to share the love here.

The continuing puzzle of the Thalassodromeus skull

Thalassodromeus sethi skull elements as figured in Witton (2013). Note how the holotype skull is a giant jigsaw with well- and ill-fitting elements. The little (drawn) jaw to the left is no longer referred to Thalassodromeus, but is now the holotype of the dsungaripterid Banguela oberlii. This photo composite was created using photographs provided by the excellent Andre Veldmeijer and Erno Endenburg.

The holotype skull of Thalassodromeus is pretty well preserved as pterosaur fossils go, but isn't quite as exceptional as it first appears (above). Though three dimensionally preserved and uncrushed, it's suffered damage in several areas and is broken into multiple pieces, some of which are ill-fitting with the rest of the skull or are missing entirely. It's a jigsaw puzzle which is complete enough to get the general picture of the skull shape, but some large areas remain open to interpretation. Pterosaur literature records that different bits of this specimen were once scattered across American research institutions and we have to hope that some of the last missing elements are still in a drawer somewhere, waiting to be reunited with the rest of the skull.

That the shape of the Thalassodromeus skull is somewhat ambiguous is evident by our history of T. sethi skull reconstructions (below). The first reconstruction - published in Kellner and Campos (2002) - is a little odd in that it shows a downturned, irregular upper jaw with a straight mandible. It also features 'classic' structures that we've come to know and love in this species: that badass 'V'-shaped chunk missing out of the back of the crest, a boss-like structure on the upper jaw, and a partly hooked mandibular tip. This reconstruction has always looked a little odd to me because I'm not sure how the animal is meant to close its mouth. A second reconstruction, which I presented in my 2013 book, was similar to the first except for showing both jaws as straight, without a downturned upper jaw. My logic was that Thalassodromeus should look something like the better known thalassodromid Tupuxuara, which has entirely straight jaws. Later, Headden and Campos (2015) presented a third interpretation, where the mandible was bent down at the base of the mandibular symphysis. Jaime Headden's (as far as I know unpublished) skull reconstruction hints at further differences from previous reconstructions, including a lack of that cool 'V' notch in the back of the crest.

Select T. sethi skull reconstructions, with my latest take at the bottom right. All three agree on some aspects of basic morphology, but there's not quite enough data to eliminate some possibilities of jaw and crest shape. Note that the 2017 skull outline is pretty conservative - the crest may have been longer and taller.

Which of these, if any, is correct? We await a comprehensive description of the skull to fully augment our understanding of T. sethi anatomy but, based on published information, it's likely that some of our earlier interpretations were erroneous. The gnarly crest shape drawn by Kellner and Campos (2002) probably takes damaged margins and missing elements too literally - this includes that awesome-looking V-shaped notch at the end, which is likely just another chunk of missing crest (this is certainly reported by colleagues who've examined the skull first hand). There's also no obvious reason why the mandible should be restored with an upturned tip. This interpretation was at least partly fuelled by an upturned jaw tip once referred to Thalassodromeus (Veldmeijer et al. 2005), but this specimen has since been considered a new genus of dsunagripterid pterosaur (Headden and Campos 2015).

It's also looking possible that - as indicated by Headden and Campos (2015) - both sets of Thalassodromeus jaws were downturned. It's difficult to be confident about any jaw reconstruction in this animal because these regions are not well represented in the holotype skull, but preserved elements of the upper and lower jaw margins imply a subtle downturn at the base of the rostrum and mandibular symphysis (and no, this isn't an effect of distortion or damage). Either Thalassodromeus had some sort of wibbly jaw shape or else it had a downturned jaw similar to azhdarchoids such as Tapejaridae* and Caupedactylus**. Whether this is convergence or further evidence of a close relationship between thalassodromids and tapejarids depends on your take on azhdarchoid interrelationships - this is still an area of disagreement that would benefit from dedicated investigation.

*of which thalassodromids - or thalassodromines - may, or may not be, a subdivision of. Ah, pterosaur phylogeny...

**I'm as confident as I can be that Caupedactylus is synonymous with my own "Tupuxuara"deliradamus. I should really write this up one day...

But hey, evidence for facial tissues and life appearance!

Thalassodromeus has some interesting features which allow us to reconstruct some aspects of its facial anatomy in detail, even in lieu of soft-tissue preservation. The crest of Thalassodromeus is marked by very conspicuous neurovascular grooves which were linked to a thermoregulatory function by Kellner and Campos (2002). They look pretty near identical to the sorts of branching grooves you find under bird beaks however (below), and my suspicion is that they're not a specialisation for controlling body temperature but simply a correlate for a keratinous sheath (Hieronymus et al. 2009). Similar grooves are seen on crestless parts of pterosaur jaws (the holotype of Serradraco sagittirostris has some especially obvious ones, for instance - see Rigal et al. 2017) as well as under the keratinous horns and beaks of animals everywhere. We don't need to imagine a unique function for these grooves just because they're on a big pterosaur crest, they're a standard variant of tetrapod skull anatomy.

Branching neurovascular networks on the Thalassodromeus crest - this is the region above the eye and posterior end of the nasoantorbital fenestra. Note the conspicuous groove crossing across the photo - this is the boundary between the premaxilla and underlying skull bones. From Kellner and Campos (2002).

Keratinous sheaths can have sharp margins which leave signature textures on the underlying skull. Bony steps or 'lips' can mark the transition to another tissue type, or a groove may form where one sheath plate abuts another. Both are evident in bird species which have beaks composed of multiple plates instead of a single keratinous covering (below), and we can look for similar features in fossil skulls to make predictions about life appearance. In Thalassodromeus we see a deep groove running along the boundary between the large premaxillary bone (the bone which makes up the jaw tip and top region of the entire crest) and the frontoparietal region (the base of the crest from the eye region backwards). Correlates for keratinous sheaths occur on both sides of this groove, so there's a chance that the crest covering was a compound structure composed of two abutting sheaths rather than one continuous one. If so, we might have been able to see this join on the live animal, just as we see the joins on the beaks of certain birds.

Gannet (Morus bassanus) skull with keratinous sheaths removed. Note the branching neurovascular impressions and deep grooves that mark the position of keratinous sheaths - we would predict a compound beak from these textures if we only knew gannets from fossils.

Can we test this idea? We could chop up our super-rare Thalassodromeus specimens to see if  histological data matches the surface texture interpretation (it's not only bone surface texture which records epidermal types - see Hieronymus et al. 2009) but I'll wager that most folks don't want the Thalassodromeus holotype carved up any more than it already is. Happily, there are other lines of data that might help us out. The first is the presence of the crest groove itself. Pterosaur skulls are normally devoid of sutures between bones because, in adults, they fuse so solidly that all trace of the original bone outlines is obliterated. Thus, the presence of a conspicuous groove in a mature Thalassodromeus specimen indicates that something unusual was happening, and influence from facial tissues is a well-known phenomenon that could explain this feature.

Schematic take on thalassodromid crest growth, from Martill and Naish (2006). The crest doesn't begin fully formed in juveniles, with the premaxillae (dark shading) having to overgrow the rest of the skull. Fun fact: my first ever PR palaeoart, now 11 years old, was to publicise this study.

A second line of support stems from studies into thalassodromid crest growth (Martill and Naish 2006). The "upper" (or premaxillary) component of the thalassodromid crest does not cover the skull in juveniles: rather, it has to overgrow the skull as the animal ages (above and below). Keratin sheaths are difficult to modify once formed because they're thick and inert (Goss 2012), so it's likely that parts of the premaxillary sheaths formed in juveniles migrated with the bone over the skull, meeting their counterparts at the skull posterior in later life. If the sheaths couldn't join once they met because they couldn't be modified or resorbed, they probably continued to grow as a compound cover, explaining the retention of an obvious groove between the two crest-forming bones. I find this idea pretty neat. Features like grooves on beaks or crests are nuances of animal appearance that are mostly lost to time but are important to characterising the appearance of living species. The idea that Thalassodromeus (and probably thalassodromids) had this feature makes them that little bit more real. Painting the images for this post certainly felt a little more like painting an animal than illustrating a hypothesis, just because of this detail.

Thalassodromid crest growth and compound keratinous sheathing, modelled by T. sethi. Note how the juvenile has an obvious 'two part' crest composition, and that the front/upper part (the premaxilla) sits on top of the posterior (frontoparietal) elements. With enough time, they form the monster-sized crest we know from big thalassodromid specimens. See Martill and Naish (2006) for more details.

Skull mechanics and lifestyle

It would be remiss to write about Thalassodromeus without mentioning its robust skull construction. The skull is proportionally wide, has especially deep jaws, a partly sealed orbit region, and the mandibular symphysis has a robust 'teardrop' cross section instead some flimsy crest. Its robustness is especially obvious when compared to the skull of the otherwise similar Tupuxuara (below), which has more typically open and airy pterosaurian cranial architecture. Thalassodromeus thus has a skull which looks like it could take a little more punishment than that of an average pterosaur, and this correlates nicely with observations that the regions for jaw adductor muscles are expanded on both the skull and lower jaw (Witton 2013; Pêgas and Kellner 2015). It's unsurprising that foraging hypotheses for Thalassodromeus have favoured forceful feeding habits such as skim-feeding (Kellner and Campos 2002) or being a predator of small-to-medium animals in terrestrial settings (Witton 2013).

Tupuxuara leonardii skull and mandible - looking pretty slender compared to the star of this post.

The possibility of downturned jaws in Thalassodromeus becomes especially interesting in light of its robust skull. Long, curving bones are a biomechanical paradox because they're weaker in compressive loading than a straight equivalent. This is, in part, because applying loads directly to both ends of a curved bone induces bending stresses even though the bone is not being bent in a traditional fashion. This is why big, slow animals tend to have straighter limb bones than smaller ones: they benefit from the increased strength of straight shafts, and they load their limbs in compression virtually all the time. From this perspective, the curved jaw of Thalassodromeus might seem like a disadvantage, being weaker under compression than that of a straight jawed animal. If striking violently at prey head on, the straight jawed species might be less likely to go home with a broken jaw.

However, curved bones are superior to straight bones at handling unpredictable, dynamic stresses. Curvature introduces predictability to stress distribution throughout a bone shaft, so they behave more reliably under a variety of loading regimes, be it compression, bending or twisting. A bone which responds to stress in the same way no matter how you deform it is easier to manage behaviourally, and to optimise mechanically, than a straight bone, and loss of raw strength created by bone curvature can be compensated for by modifying cross sections, shaft diameters and internal reinforcement (Bertram and Biewner 1988). These attributes have not been ignored by evolution and, in fact, most animal limb bones are curved to some degree to take advantage of these effects (Bertram and Biewner 1988). The superior compressive performance of a straight bone may not be as advantageous as the reliability and potential all-round stress resistance of a curved variant so, in simple terms, if you're planning some crazy stunts with your long bones, you want curved bone shafts, not straight ones.

A curved jaw thus complements the strong skull and jaw muscles of Thalassodromeus. If Thalassodromeus used foraging mechanics which were forceful or violent - such as catching big or powerful prey types, or using its beak to batter or tear at other animals - a curved beak may have served it well. This jaw shape - assuming we've interpreted it correctly, remember - could be further evidence of foraging habits at the more explosive and exciting end of the pterosaur ecological spectrum. Exactly what Thalassodromeus did for a living remains unknown, but it's hard not to compare these cranial features with other ideas of robust, terrestrial azhdarchoid predators - maybe this 'large pterosaur predator' niche has a longer roster than we've traditionally thought.

Hypothesis B: spinosaurids were allergic to curved jaws. Hey, it could happen.

Thalassodromids and their azhdarchoid kin are exceptionally interesting animals and we could probably talk about them all day, but we'll have to stop there. Coming soon: pterosaurs from the other end of the pterodactyloid spectrum, or a return to the world of extinct mammals. Probably.

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This blog is sponsored through Patreon, the site where you can help online content creators make a living. If you enjoy my content, please consider donating $1 a month to help fund my work. $1 might seem a meaningless amount, but if every reader pitched that amount I could work on these articles and their artwork full time. In return, you'll get access to my exclusive Patreon content: regular updates on research papers, books and paintings, including numerous advance previews of two palaeoart-heavy books (one of which is the first ever comprehensive guide to palaeoart processes). Plus, you get free stuff - prints, high quality images for printing, books, competitions - as my way of thanking you for your support. As always, huge thanks to everyone who already sponsors my work!

References

• Bertram, J. E., & Biewener, A. A. (1988). Bone curvature: sacrificing strength for load predictability?. Journal of Theoretical Biology, 131(1), 75-92.
• Headden, J. A., & Campos, H. B. (2015). An unusual edentulous pterosaur from the Early Cretaceous Romualdo Formation of Brazil. Historical Biology, 27(7), 815-826.
• Hieronymus, T. L., Witmer, L. M., Tanke, D. H., & Currie, P. J. (2009). The facial integument of centrosaurine ceratopsids: morphological and histological correlates of novel skin structures. The Anatomical Record, 292(9), 1370-1396.
• Humphries, S., Bonser, R. H., Witton, M. P., & Martill, D. M. (2007). Did pterosaurs feed by skimming? Physical modelling and anatomical evaluation of an unusual feeding method. PLoS Biology, 5(8), e204.
• Goss, R. J. (2012). Deer antlers: regeneration, function and evolution. Academic Press. 
• Kellner, A. W., & de Almeida Campos, D. (2002). The function of the cranial crest and jaws of a unique pterosaur from the Early Cretaceous of Brazil. Science, 297(5580), 389-392.
• Martill, D. M., & Naish, D. (2006). Cranial crest development in the azhdarchoid pterosaur Tupuxuara, with a review of the genus and tapejarid monophyly. Palaeontology, 49(4), 925-941.
• Pêgas, R. V., & Kellner, A. W. (2015). Preliminary mandibular myological reconstruction of Thalassodromeus sethi (Pterodactyloidea: Tapejaridae). Flugsaurier 2015 Portsmouth, abstracts, 47-48.
• Rigal, S., Martill, D. M., & Sweetman, S. C. (2017). A new pterosaur specimen from the Upper Tunbridge Wells Sand Formation (Cretaceous, Valanginian) of southern England and a review of Lonchodectes sagittirostris (Owen 1874). Geological Society, London, Special Publications, 455, SP455-5.
• Veldmeijer, A. J., Signore, M., & Meijer, H. J. (2005). Description of two pterosaur (Pterodactyloidea) mandibles from the lower Cretaceous Santana Formation, Brazil. Deinsea, 11(1), 67-86.
• Witton, M. P. (2013). Pterosaurs: natural history, evolution, anatomy. Princeton University Press.

Triceratops horridus with some crazy long and curving brow horns. Just speculation, right? Surprisingly, maybe not...

For palaeoartists, animals with flamboyant headgear are among the most rewarding to render, but it's not only the bony aspects of their cranial ornaments that we have to pay attention too. Animal headgear is covered with various amounts of soft-tissue that, in extreme cases, can dramatically augment the shape of the underlying bony features. The headgear of living species has a spectrum of soft-tissue coverings from nothing at all (mature deer antlers), to relatively thin dermal tissues (giraffe ossicones), through to hard keratin sheaths that can add significant depth and length to a horn or crest (most other animal horns). This excellent breakdown of a bighorn sheep face by Aaron Drake of Colorado State University (uploaded by Simpleware Software Solutions) gives a pretty good idea of how much tissue extreme keratin sheaths can add to the underlying skull.

Not all horns are augmented to the extent seen in bighorn sheep, but even modestly proportioned keratin sheaths can add a lot of bulk, length and characteristic geometry to horn tissues. Thus, anyone hoping to accurately predict the appearance of ancient horned animals should want to predict the shape of their horn sheaths along with understanding the skull geometry. This isn't easy because, though incredibly tough and resistant, keratin sheaths are still prone to decay and rarely fossilise.

Researching horn growth for an upcoming book project has made me wonder if horn sheath shape might be more predictable than we've traditionally thought, however. Horn sheath growth mechanics are relatively simple, closely related to bone shape, and constrained by the properties of heavily keratinised tissues. They're also fairly universal across across tetrapods - the same processes that make a goat horn will make the enormous keratin sheath of a skimmer jaw, for instance. These properties might allow insights into sheath shape in fossil species even when the sheath is not preserved. So what aspects of horn sheath growth might allow this, and how could we transfer them to fossil animals?

Growing horn sheaths in living animals

Keratin sheaths are dead tissue with their only living components being the cells that synthesise the keratin at the horn core/sheath interface (e.g. at the inner surface of the horn soft-tissues, see diagram, below). Because no living tissue reaches the outer horn surface, they cannot grow by adding tissue to the tip. Rather, they grow by internal accumulation of keratin layers, each new deposit displacing the older sheath from the bony core. This creates a stack of keratin cones, with new cones growing at the base and causing the horn tissues to lengthen. Continuous internal deposition and displacement of old material is what creates the soft-tissue horn extension, as each new keratin layer shoves the older material a little further from the bony tip. This makes the tip of a keratin horn the oldest part of the sheath, and in many bovids the tips are many years old. Conversely, the youngest part of the horn tissues are located at the base. As we discussed in a recent post about the horns of Arsinoitherium, this growth mechanism binds the internal horn tissues in the overlying sheaths, limiting their ability to change size or shape. Changes in size or curvature can only be achieved by displacing the older horn layers, but complicating the horn shape - say, by branching the tip - is impossible unless the sheath is shed, pronghorn-style. The sheath itself can't be modified after deposition either, on account of no living tissue reaching it. Thus, old sheaths permanently maintain the size and geometry they were created with.

Stylised bovid horn growth, heavily modified from Goss (2012).

This growth mechanic presents three important points relevant to predicting the shape of fossil horn sheaths. The first is that sheath tissues are synthesised directly over the horn core, effectively making the internal sheath margin a cast of the bone at the time it grew. The second is that the shape of new keratin layers are constrained by the keratin sheaths that preceded them. They can't deviate too radically from the overlying horn shape and the horn core of the emerging layer should mostly nestle into margins of the older one. The third is that horn extensions are not simply exaggerations of their contemporary horn core, but a keratinous record of the horn history. Geometry exhibited by the earliest growth stages is maintained in the extending sheath regardless of later changes to the horn core morphology, and only periodic shedding or heavy abrasion are likely to alter this.

This being the case, could ontogenetic changes in horn cores provide insight into the sheath shape of fossil animals? If bone shape translates to keratin sheath shape, and sheath shape dictates the horn extension profile, then a growth series of bony horn anatomy may allow us to reconstruct horn keratin accumulations that are otherwise lost to decay. Horn core profiles give us a 'cast' of the inner sheath margin for that growth stage, and we can fit these into the margin of the preceding sheath layer (which, of course, can be deduced by the shape of a ontogenetically preceding horn core). Building a stack of nestled horn core profiles creates something akin the bovid horn diagram above and tells us something of how keratin layers were accumulated for that horn shape. The very tip of the horn sheath is lost to time because we cannot predict external appearance from horn core casts (they only represent the internal structure) but if the youngest animal in a growth series is suitably juvenile, we probably aren't missing much.

As proof of concept, I've taken the horncore outlines from the schematic bovid horn above and attempted to recreate the horn shape. Stacking them was achieved by simply eyeballing the margins, trying to fit the horn core outlines together as tightly as possible without their margins overlapping. Here's how it turned out...


I don't think that's too bad. It's not perfect, but it gives a pretty good idea what's going on with the actual horn. This method is very simple, but - as outlined earlier - keratin horns are simple, so we might not need a particularly complex method to predict their shape. But you're not here to talk about ram horns: what happens when we apply this idea to a fossil animal with a well-known growth series, and how do the results compare to our conventional means of reconstructing horn sheaths in fossil taxa?

Step forward, Triceratops

Triceratops growth series from Horner and Goodwin (2006). Both species of Triceratops are included here, but the generalities of this growth sequence are thought to apply to both. Say, that brow horn curvature looks pretty changeable - what would that mean for horn sheath shape?

The super-famous horned dinosaur Triceratops is a great animal to explore this idea with. It's known from dozens of specimens representing a range of ontogenetic stages, from small juveniles to giant adults (above, Horner and Goodwin 2006 - and no, the adults in question here are not Torosaurus). Like the horns of other ceratopsids, Triceratops brow horns have well-developed epidermal correlates for keratinous sheaths (oblique foramina and anastomosing neurovascular channels - Horner and Marshall 2002; Hieronymus et al. 2009) and these textures are present in the smallest known skulls, indicating that most or all their life was spent with sheathed brow horns (Goodwin et al. 2006). Confirmation of a horn sheath comes from poorly-preserved soft-tissues found on some Triceratops horns (Farke 2004; Happ 2010).

Triceratops skulls underwent pretty major changes as they grew, including complete reorientation and allometric scaling of the brow horns. In juveniles these curve backwards, but in big adults they arc forwards (Horner and Goodwin 2006). Typically, artists have assumed that the keratin sheaths covering these horns changed shape with them. Even pros, such as Greg Paul (2016), who have stressed that the keratin sheath should extend the horn shape, render the sheaths as more-or-less reflecting the underlying horn core of a given growth stage, without any hangovers from a previous iteration of horn shape. Whether intentional or not, the implication here is that the horn sheath was dynamic - capable of changing as the animal grew.

....just like this. Note how the brow horns of this Triceratops group are clearly changing shape as the animals increase in size, but that the keratin sheaths don't reflect any earlier horn history. Hmm. Say, do you know this image is on the front of my 2018 calendar?

The model outlined above conflicts with this traditional take, however. If we assume that the horn extension was composed of a series of retained keratin sheaths, and using Horner and Goodwin's (2006) ontogenetic sequence as a basis, the resultant horn shape is pretty surprising. Stacking horn cores in the juveniles sees those recurved shapes pushed off the horn core to extend and extenuate the curve strongly, to the point where the horn tip even points posteriorly at one stage (below). As the horn base tips forward on the approach to adulthood, these arcing tips rotate with them, creating a long, elaborate set of horns which curved twice: once at the tip, and again, but inversely, at the base. If the Triceratops in this model retained the full history of their horn sheaths into adulthood, the result would be pretty fantastic: very long horns where the tips pointed 90° away from the point of the horn core. Yowsers - that's quite different from our traditional 'just make it pointier' approach.

Stacking Triceratops horn cores, mimicking how living animal keratin sheaths grow, suggest the keratinous extension of the brow horns was strongly curved even in adult animals. As in the mock bovid horn above, the horn cores were stacked simply by trying to make them fit as neatly as possible.

Which is more likely: twirly horn sheaths or the more conventional, 'dynamic' sheaths? Where morphing horn sheaths immediately lose points is their requirement for the inert keratin horn tissues to react to each horn core shape, as well as for the horn sheath history to continuously disappear. Modern horn sheaths just don't grow like this: their extensions only exist because the old keratin tissues hang around, and we have to ask how the extending sheaths are created in our 'dynamic' sheath model. There are perhaps two ways we could attain morphing sheaths: the first is through continuous eradication of old sheath material, allowing new keratin to grow over the horn core without being obscured by previous sheath layers. This might have been achieved by Triceratops shedding and regrowing sheath extensions, or by abrading outer sheath tissues away. The second is that the horns weren't covered in one sheath but several interlocking plates, like the beaks of some birds, which might allow for jimmying and reconfiguration of the horn tissues through growth without adding lots of material to the end.

Let's consider shedding first. It's possible that at least some layers of Triceratops horns were shed because exfoliation is common on keratin sheaths in living species. For instance, puffins shed the outer layer of their beaks annually, and bovids exfoliate outer layers of their horns once or twice in their lifetimes (O'Gara and Matson 1975; Goss 2012). The fact that only a superficial layer of tissue is lost prevents the sheath being significantly altered however: exfoliation alone would probably not give us particularly 'dynamic' horn sheaths.

Constant reshaping of horn tissues might be plausible if Triceratops could regularly shed and regrow the horn sheath, as performed by pronghorns. Unfortunately, these mammals show us that detecting this growth mechanic in fossil species is challenging, however. Despite their unusual habit of regrowing an entirely new sheath each year, pronghorn horn cores have similar textures to those of animals with permanent sheathing (Janis et al. 1998). There are some differences, but they're subtle. O'Gara (1990) reported that pronghorn horn cores have annually variable properties, alternating between a spongy, relatively rounded horn core when the sheath is growing, and a smooth-textured, sharper horn core at peak sheath hardness (O'Gara 1990). It's pretty well established that dinosaur skeletons grade from spongy, rounded bones to smoother, sharpened bones as they aged, so perhaps variation in texture and shape of Triceratops horns that broke this pattern could indicate horn shedding - provided these differences could be distinguished from ontogenetic or intraspecific factors. I'm not aware of any evidence of this kind, despite the frequency in which Triceratops skull bone texture is commented, but I also don't know that anyone has specifically looked for this variation yet.

Lovely, lovely epidermal correlates on the skull of Triceratops prorsus illustrated in Hatcher (1907). Note that there's no divide between the correlates on the brow horn and surrounding skull - might we expect some sort of dividing sulcus if the horn sheath was routinely cast? From Wikimedia, uploaded by Biodiversity Heritage Library, CC BY 2.0.

A more illuminating insight may be that the correlates for Triceratops horn keratin are continuous with the epidermal correlates of the face (above). Horner and Marshall (2002) noted that the horn correlates for keratin sheathing extend over virtually the entire face - including the back of the frill (this is why so many Triceratops reconstructions have smooth 'face shields' nowadays). However, what's not seen on Triceratops horns is a boundary dividing the face sheath and a hypothetical temporary horn sheath, as might be expected where two keratinous sheaths meet (I'm assuming that the entire face shield wasn't shed annually either (palaeoartists: exfoliating/shedding Triceratops face - go!) - that's not a discussion I want to get into here).

A last, more arm-wavy point against horn shedding is that it is not at all common among living animals, possibly not even being present in some close pronghorn relatives (Janis et al. 1998). If Triceratops did shed its horns, it would be part of a club with very few members. This isn't a particularly scientific argument, but we have to concede that permanent horn sheaths are - by some way - far more common than ephemeral ones, and probably the 'default' condition for horned animals. Maybe we should assume permanence until there's good reason to think otherwise?

Could wear and abrasion create our morphing, dynamic horn sheaths in Triceratops? It's certainly true that keratin horns can be worn down, sometimes considerably. Bighorn sheep, for instance, can wear away years of horn growth in a behaviour known as 'brooming', but the results do not look like our palaeoart - in other words, they don't look like these sheep stuck their horns in a pencil sharpener. Nor do they echo the shape of the underlying skeleton. Instead, the ends are blunt, frayed and fractured (below). Any Triceratops that removed horn keratin through abrasion would presumably adopt a similarly 'sawn-off' appearance, and lack neat, pointed tips.

The broomed horns of a bighorn sheep (Ovis canadensis) - notice that they're heavily and deliberately worn at the tips, but they aren't shaped into fine points. From Wikimedia, uploaded by Lake Mead NRA Public Affairs, CC BY-SA 2.0.

Might a compound horn sheath be a route to horn sheath dynamism for Triceratops? Some readers may recall that we discussed compound keratin sheath covers last month and that they typically have deep grooves between abutting sheets. We don't see grooves of this nature on Triceratops skulls despite the very obvious rugosity profile created by the epidermal tissues, so I think we have to reject this hypothesis outright. The coverage of Triceratops horn core epidermal rugosities are pretty near identical to what we see on the horns of animals like cattle or goats, and I think we have to assume they indicate a similar, all-encompassing sheath morphology.

If Triceratops horns couldn't be renewed or take advantage of a more complex sheath arrangement, the likelihood of dynamic Triceratops horn sheaths is probably low. But does this idea of continuous sheath growth and twirly horns fare better under scrutiny? It seems to pass some basic tests, at least. The Triceratops brow horn outlines fit together pretty well with only a little displacement of the preceding horn layer, which is just what in see in modern horn growth, and the fact that their horn profiles don't change suddenly is consistent with them being perpetually constrained by layers of hard tissue. The predicted Triceratops sheath profile it is unexpected, but not beyond anything we see in living animals. And it scores points generally for being a simple model that is grounded in a well-understood aspect of living animal biology, in not needing to explain the loss of sheath tissue, and for factoring data we know is relevant to horn growth in living animals. I'm not saying this model is correct, but I am thinking that explains and fits our available data better than the dynamic sheath concept.

Of course, there are still lots of caveats. Remember that the model here is rough, being based on a generic Triceratops dataset and not the growth regime of a single species. The growth series outlined by Horner and Goodwin (2006) is a good general illustration of Triceratops growth, but results might vary if we restricted the data to a single species. My illustrations do not assume any exfoliation or tip abrasion, and we still don't have any idea what the external sheath morphology - including the presence of absence of ridges, spirals and bosses - might have been like. My attempt to stack the horn core profiles has also assumed minimal sheath thickness. If the sheath was thicker, the arcs of the horn could be stretched out over longer distances. So if you're buying this concept, remember that the horn shape proposed is only a general one - it's more in keeping with our understanding of sheath grow in modern animals, but it's still quite sketchy.

So...

Perhaps the take-home message here is not, however, that Triceratops might have had loopy horns, but that there might be more to consider about fossil horn sheaths than we've assumed. Our discussion of dynamic horn sheaths does not just apply to Triceratops: artists take this approach with most horns and spikes in palaeoart, and it's clearly at odds with how most animals grow keratin sheaths today. But maybe this isn't just a topic for artists to ponder. There's potentially scope for a real study here and, seeing as fossil horn shape has a lot of functional significance, predicting sheath morphology would be a useful aid to predicting ancient behaviour. This needn't be restricted to horned dinosaurs, or even just horns, either: keratin sheaths on plates, spikes and so on grow in a similar way, and there's not reason this technique couldn't be used on other body parts, if validated. Moving this from food-for-thought-blog post to genuine science would require testing on modern species, perhaps through reconstructing living animal horns, to see how well it holds up. Recreating a schematic, 2D goat horn sheath using this method is fine, but real-world tests - especially using 3D horn casts, not just 2D drawings - might be more challenging. In the meantime, I'm curious to know what others think of all this - the comment field is open below...

"Hello, I'm Triceratops. I'll be your odd-looking concluding dinosaur reconstruction for this evening."

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References

• Farke, A. A. (2004). Horn use in Triceratops (Dinosauria: Ceratopsidae): testing behavioral hypotheses using scale models. Palaeontologia Electronica, 7(1), 1-10.
• Goodwin, M. B., Clemens, W. A., Horner, J. R., & Padian, K. (2006). The smallest known Triceratops skull: new observations on ceratopsid cranial anatomy and ontogeny. Journal of Vertebrate Paleontology, 26(1), 103-112.
• Goss, R. J. (2012). Deer antlers: regeneration, function and evolution. Academic Press.
• Happ, J. W. (2010). New evidence regarding the structure and function of the horns in Triceratops (Dinosauris: Ceratopsidae). In: Ryan, M. H., Chinnery-Allgeier, B. J. & Eberth, D. A. (Eds.) New Perspectives on Horned Dinosaurs: The Royal Tyrrell Museum Ceratopsian Symposium. Indiana University Press. pp. 271-281.
• Hieronymus, T. L., Witmer, L. M., Tanke, D. H., & Currie, P. J. (2009). The facial integument of centrosaurine ceratopsids: morphological and histological correlates of novel skin structures. The Anatomical Record, 292(9), 1370-1396.
• Horner, J. R., & Goodwin, M. B. (2006). Major cranial changes during Triceratops ontogeny. Proceedings of the Royal Society of London B: Biological Sciences, 273(1602), 2757-2761.
• Horner, J. R., & Marshall. C. (2002). Keratinous covered dinosaur skulls. Journal of Vertebrate Paleontology 22(3, Supplement):67A.
• Janis, C. M., Manning, E., & Ahearn, M. E. (1998). Antilocapridae. In: Janis, C. M., Scott, K. M., & Jacobs, L. L. (Eds.). Evolution of tertiary mammals of North America: Volume 1, terrestrial carnivores, ungulates, and ungulate like mammals (Vol. 1). Cambridge University Press
• O’Gara, B. W. (1990). The pronghorn (Antilocapra americana). In: Bubenik, G.A. & Bubenik, A. B. (Eds). Horns, pronghorns, and antlers: evolution, morphology, physiology, and social significance, Springer-Verlag. pp 231-264.
• O'Gara, B. W., & Matson, G. (1975). Growth and casting of horns by pronghorns and exfoliation of horns by bovids. Journal of Mammalogy, 56(4), 829-846.
• Paul, G. S. (2016). The Princeton field guide to dinosaurs. Princeton University Press.

Brain 1: "Right, you need an image for your tyrannosaurid facial tissue post." Brain 2: "OK, here're some Tyrannosaurus rex in a really dark and back-lit scene. Their faces are in shadow, and you can't really see the features." Brain 1: "This is perfect. After all, only losers want to see the faces of animals in posts about facial tissues." Brain 2: "Exactly. Hey, since when did I have two brains?" Brain 3: "Beats me."

Discussing the craniofacial tissues of tyrannosaurid dinosaurs is the palaeointernet equivalent of lighting a match in a straw-filled barn - the slightest spark of opinion spawns a 100-strong comment field about extra-oral tissues, tooth exposure, rictal tissues, facial skin depth and a number of other topics. But despite this keen popular interest, there's been relatively little academic study into tyrannosaurid facial tissues, perhaps because their soft-tissues mostly remain unrepresented in the fossil record. Happily, close examination of tyrant skulls reveals a number of textures and rugosity profiles which were almost certainly created by bone-skin interaction, so we can form some idea of their life appearance even without soft-tissue specimens. The first detailed attempt at interpreting tyrant cranial rugosities was published last year by tyrannosaur expert Thomas Carr and colleagues (Carr et al. 2017 - you might also know Thomas by his super-comprehensive blog Tyrannosauroidea Central). This widely publicised paper proposed a number of hypotheses about the face of Daspletosaurus horneri: that the sides of the jaw were adorned with crocodile-like 'facial scales'; that various scales, dermal armour and cornified sheaths adorned the nasal and orbital region; and that it lacked lips (not explicitly stated in the paper, but restored as such in an illustration and touted in the paper's PR). The idea that tyrannosaurids may have had crocodylian-like facial tissues has since generated a lot of discussion online, some in favour, some against, and as someone increasingly looking at epidermal correlates for palaeoartistic purposes, I thought this topic was worthy of a blog article: are tyrannosaurid jaws really croc-like enough to assume comparable skin types?

(An important caveat before we start this discussion is that the following is based on tyrannosaurids generally, not D. horneri specifically, because the horneri study does not include photographs of its alleged epidermal correlates. The D. horneri paper describes them very well (see Carr et al. 2017, supplementary data), but it's difficult to evaluate them without images of the bone surfaces themselves. Dave "Tyrannosaur Chronicles" Hone needs a shout out here for sharing his expertise and extensive image library of tyrant fossils as I prepared this post - though I have some experience with tyrannosaur bones and their interpretation, this article has been considerably improved by his involvement.)

Tyrannosaurids and crocodylians: face off

An obvious place to begin this discussion is crocodylian facial structure. Crocodylian skulls are so familiar that it's easy to forget how distinctive they are among modern animals, and I don't think it's widely known that their skin plays a significant role in shaping their skull tissues. Crocodylian jaw bones have incredibly high numbers of foramina, with averages of 100 in each major jaw bone (premaxilla, maxilla and dentary) and over 1000 in each bone in some specimens (Morhardt 2009). These openings are the loci around which gnarly ridges and tubercles grow by a process of dermal ossification: tissues from the skin are turned to bone and build up the sculpting on the skull surface (Grigg and Kirshner 2015; de Buffrénil et al. 2015). Simultaneously, the bone immediately surrounding the foramina is resorbed, enhancing the rugosity pattern further and creating that highly distinctive, deeply pitted and grooved crocodylian skull texture (de Buffrénil et al. 2015). This restructuring can be extensive and, over ontogeny, crocodylian snout surface area can increase by as much as 20% (de Buffrénil et al. 2015). That's a major reworking of the superficial bone of the skull, and their skin has a major role in its development.

Skull of a mature American crocodile, Crocodylus acutus, demonstrating that classic crocodylian skull texture. Cropped from public domain Wikimedia image by Daderot.

Among living tetrapods, only some turtles and a couple of geckos show a comparable degree of sharply-defined cranial sculpting (Evans 2008; de Buffrénil et al. 2015) but, among extinct taxa, stem-tetrapods, temnospondyls, parareptiles and many crocodylomorphs present analogous cranial conditions (Witzmann et al. 2010; de Buffrénil et al. 2015). Studies show that temnospondyl skulls developed their sculpting via a similar mechanism of ossifying dermal tissues (Witzmann et al. 2010), perhaps indicating croc-like skin properties in these animals, too. Until recently it was thought that crocodylian facial skin was scaly, but new research shows that it is actually a sheet of toughened skin which cracks through growth, creating a scaly appearance, but not true epidermal scales akin to those seen in lizards (Milinkovitch et al. 2013). Regardless of whatever other conclusions are drawn here, this has to be a minor amendment to Carr et al.'s (2017) interpretation: if tyrannosaurids (or any other extinct animal) have croc-like textures on their jaw bones, we should be visualising tight, tough skin, not epidermal scales.

Juvenile alligator, Alligator mississippiensis, showing virtually crack-free facial skin - it's only adults that develop the extensively cracked, superficially 'scaly' faces. Photo by Joxerra Aihartza, from Wikimedia, FAL 1.3.

Whether tyrannosaurid jaws are truly crocodylian-like is open to question, however. Carr et al, (2017) are clear that they consider tyrants and crocodylians jaws as identical in superficial appearance ("The texture in crocodylians is identical to that of tyrannosaurids, except that the entire face of crocodylians is coarse in texture" - p. 21; Supplementary information to Carr et al. 2017) but I disagree: there are a number of ways in which they differ and, given the link between crocodylian skull development and dermal tissues, these differences may be critical to our considerations of facial anatomy. Many of these contrasts pertain to jaw foramina, which we know are important in defining crocodylian cranial sculpting (de Buffrénil et al. 2015) and may have a deeper relationship with jaw tissue properties (Morhardt et al. 2009; Hieronymus et al. 2009).

Firstly, although tyrannosaurids have elevated numbers of jaw foramina compared to other dinosaurs, their numbers are, on average, significantly lower than those of crocodylians (Morhardt 2009). No tyrannosaurid jaw bone reported by Morhardt (2009) exceeds 81 foramina, which is high for a dinosaur, but still short of the crocodylian average, and well below the 1000+ figure reported for some croc jaws. Interestingly, data in Morhardt (2009) suggests that foramina numbers weakly correlate to jaw size: the longer a jaw is, the more foramina it generally has. This trend is particularly well shown in her tyrannosaurid sample but seems true of other fossil and extant animal groups as well, and might also be reflected in ontogeny (smaller Tyrannosaurus have fewer foramina, on average, than large ones). The cause behind this trend seems to be elusive at present - might it reflect a change in tissue type with age (Morhardt 2009)? does it reflect demands of supplying an absolutely larger jaw with nervous and vascular tissues? - but whatever the reason, it implies that we should consider foramina frequency proportionate to jaw size when analysing rugosity profiles. Under this metric, foramina values in crocodylian jaws are even more impressive as, compared to some extinct animals, their skulls are of middling size. By contrast, the slightly above-average foramina counts of even the largest tyrannosaurines seem less significant because, even with extreme jaw size, they don't attain a value comparable to a much smaller alligator. If we remove size from our consideration by comparing similarly-sized tyrant and croc jaws, we find they are worlds apart in terms of jaw perforation. Indeed, the foramina values of smaller tyrants are nothing special - they are comparable to most other similarly-sized tetrapods (Morhardt 2009). Presumably, this explains why - as many internet conversations have pointed out - tyrannosaurid jaws simply don't have that same obvious, pitted surface as those of crocodylians.

Further differences might be noted in relative foramina sizes. Those foramina occurring high on tyrant snouts - such as at the top of the maxilla - are much smaller than the broader, obviously deep labial foramina paralleling the jaws (Brochu 2003; Carr et al. 2017). In crocodylians however, jaw foramina seem to have a lower size range. Foramina shape and size is an important consideration for facial tissues (Hieronymus et al. 2009) and this might imply different tyrant facial tissues over the side of the snout vs. those at the jawline, whereas the more uniform foramina sizes of crocodylians are entirely consistent with their homogeneous jaw skin.

Schematic drawing of Tyrannosaurus skull FMNH PR2081 (the specimen better known as 'Sue') showing the distribution and (somewhat conservatively) size differences in jaw foramina. This huge skull is said to be one of the most rugose Tyrannosaurus skulls known (Brochu 2003), but it fails to meet the high foramina numbers, sculpting extent and uniform foramina size of mature crocs. Image from Brochu (2003).

A related issue concerns a possible link between extra-oral tissues and foramina counts. Morhardt (2009) noted that, as a general rule, extant animals with average foramina counts below 50 in each jaw bone have tooth-covering extra-oral tissues; that those above 50 but below 100 have immobile facial tissues; and only those with 100 or more are reliably excluded from having lips or other means of tooth coverage. Average tyrant jaw foramina counts are well below that upper threshold for exposed teeth so, by this metric, they should have lips, and would not look like bipedal crocodiles. This might match what we're seeing with tyrant foramina size: perhaps those large labial foramina are something to do with nourishing and innervating extra-oral tissues, while those on the side of the snout need only access the overlying skin. There are some complications to Morhardt's data (if anyone is looking for a PhD project, a more extensive follow up would be terrific) but, at face value, her research does not support crocodile-like facial tissues for tyrannosaurids.

Finally, we can observe that the ontogeny of tyrannosaurid skull textures is not at all crocodylian-like. Tyrants do have some sculpting on their jaw bones and, as with most reptiles, these become better defined with maturity (e.g. Evans 2008; de Buffrénil et al. 2015). However, even the most rugose tyrannosaurid skulls do not match the complex and sharply pitted rugosity patterns of mature crocodylians (e.g. Osborn 1912; Carr et al. 1999; 2017; Brochu 2003; Hone et al. 2011). Given that ossifying facial skin is a direct factor in jaw bone sculpting in crocodylians, the lack of comparable development in tyrannosaurids is a blow to the idea that their faces bore the same dermal regime. Histological examination of tyrannosaurid jaw bones for might have further insight here, as the resorption/remodelling pattern might reveal details about bone/dermal interactions (Witzmann et al. 2010; de Buffrénil et al. 2015) but, for now, this inconsistency seems to be a big hole in the idea that tyrannosaurids had crocodylian-grade facial tissues.

Does the tyrannosaurid EPB help here?

Collectively, these points seem to suggest that tyrant jaws are not as croc-like as argued, and that it's not a given that the two groups had similar facial tissues. A counterargument to this is that crocodylians are the best tyrant analogues in their extant phylogenetic bracket (EPB), and thus give us our best, most phylogenetically informed insight into tyrannosaurid faces. Indeed, the croc-snouted tyrant hypothesis was informed primarily by comparisons with taxa from the tyrannosaurid EPB - specifically the skulls of birds and alligators (Carr et al. 2017) and, sure, crocs and tyrannosaurid jaws may not be exactly alike, but they're undeniably more similar to each other than either is to a bird. Might we concede that the comparisons aren't perfect, but that this is simply the best we can do without violating the tyrannosaurid EPB?

Our issue is that, while the EPB is a terrific method for predicting ancient anatomies, it really struggles with the complexity of archosaur facial tissue evolution, perhaps to the extent of being redundant. One major issue is that we can be near certain early archosaurs had neither croc- or bird-like facial tissues because no species representing the earliest phases of archosaur evolution have comparable skin-influenced jaw textures (see Nesbitt et al. 2013, and papers therein). Rather, we only see these features developing in relatively crownward archosaur groups, implying independent development of their respective facial anatomies well after the croc-bird split. This being the case, the common archosaur ancestor must have had a different set of facial tissues, and the facial anatomy of extant archosaurs may tell us little about the faces of Mesozoic dinosaurs.

Like crocodylians, birds have jaws with surface textures shaped by their overlying skin: networks of branching neurovascular canals and oblique foramina underlie cornified sheaths (their beaks). The prominence of such jaw rugosities in living archosaurs allows us to predict the facial condition of fossil archosaurs and stress test the tyrannosaur EPB, and it doesn't seem to hold up well. This skull is a marabou stork (Leptoptilos crumenifer), photo by me.

A second major issue is evidence that living archosaur faces don't reflect tissues known from their fossil cousins. In addition to tight facial dermis and cornified sheaths, a plethora of fossil evidence show that fossil archosaurs had faces with epidermal scales, projecting skin tissues (e.g. pterosaur crests) armoured dermis, and cornified pads (Frey et al. 2003; Hieronymus 2009; Hieronymus et al. 2009; Carr et al. 2017). These go well beyond the anatomical range implied by the EPB and show that fossil archosaur faces sometimes had more in common with non-archosaurs than their closest extant relatives. We must remember that the EPB is a predictive method which should be applied where no other data is forthcoming: in this case, we have enough fossil data to show that our EPB predictions are problematic, and that we can't rely on it for insight into tyrannosaurid faces. I'm hardly the first to suggest EPB approaches don't help discussion of non-avian dinosaur faces (e.g. Vickaryous et al. 2001; Knoll 2008), but these points are worth repeating in this context: I don't think the EPB is a compelling supporting argument for a croc-faced tyrannosaurid.

So, if not croc-like, what might be happening here?

If croc-skinned tyrant snouts are problematic, what are our other options? Our discussion above really only pertains to the maxillary region of tyrannosaurid snouts and, for the rest of the skull, I think Carr et al. (2017) nailed it: what I've seen of tyrannosaurid skulls suggests the orbital region and skull roofs were covered in cornified sheaths, armoured dermis and large scales. There seems to be quite a bit of variation in these tissues, with some taxa having more defined scale correlates over the nasals than others, as well as differences in elaborations of the hornlets above the eye. In all likelihood, different tyrant species would be highly recognisable in life by the development of scales, armour and horn across the top of their faces. These armoured tissues are entirely consistent with what we understand of tyrannosaurid behaviour: if you were being routinely bitten about the face by another tyrannosaur, you'd want some protection too (see opening image).

Dorsal view of the snout of a red river hog (Potamochoerus porcus). These pitted, grooved bone textures are fairly widespread across tetrapod skulls and don't seem to correlate to any one skin type, but might indicate the presence of tough, well-cornified skin (these hogs wrestle with their faces, so need protected snouts). Note the projecting rugosities on the side of the snout and on the ascending maxillary projection - these anchor vast skin projections in life. Red river hog skulls are awesome. Photo by me.

But what of that maxillary portion of the snout - the lateral region suggested as being crocodile-like? The surfaces of tyrannosaurid maxillae are pretty complex with a hierarchy of rugosity profiles (Carr et al. 2017). Very obvious features include many pits and short, branching neurovascular grooves: these might not necessarily indicate of particular tissue type in themselves, but are often associated with a well-cornified, tough epidermis (above). The high number of foramina in tyrant maxillae implies immobile facial tissues (Morhardt 2009), which I guess we probably expected in a reptile anyway.

Holotype maxilla of Zhuchengtyrannusmagnus: check out that network of elliptical depressions bordered by raised regions. Note how they terminate about a few centimetres above the line of labial foramina - we'll come back to this in a moment. From Hone et al. (2011).

Underlying these pits and grooves are a series of large, elliptical shallow depressions surrounded by low ridges (above). These vertically-aligned structures are found in many tryannosaurids and are especially obvious in large tyrannosaurines like Tyrannosaurus, Tarbosaurus and Zhuchengtyrannus. You can see them easily in museum mounts, even from across the room. Some taxa have single rows of these structures below the antorbital fenestra (Tyrannosaurus), but others have tessellating networks of depressions and ridges that extend to the top of the maxilla (Tarbosaurus), terminating beneath the scaly region overlying the nasal bone. They're unusual structures which are almost certainly epidermal in origin: they're in a place where epidermal correlates often form; are more pronounced in mature individuals; are regularly and consistently arranged across the surface of the skull; and are not associated with any pneumatic or neurovascular openings. They broadly recall the 'hummocky' rugosity profile seen under epidermal scales (Hieronymus et al. 2009) and, if so, the convex, ridged areas probably underlay vertically aligned scales, or rows of scales. Some tyrant skulls, such as the especially rugose Tyrannosaurus skull AMNH 5027, have especially sharp and rugose ridges which, to me, recall the facial ridges of certain iguanine lizards: specifically, anoles, chameleons and basilisks. These are often quite rugose and sculpted, but smoother, more tyrannosaurid-like conditions exist in a number of species (I'm thinking of things like helmeted basilisks and smooth chameleons). Prominent, ornamental rows of relatively large and often colourful scales overly these structures in these iguanines and I wonder if the same was true for tyrants. Alternative hypotheses, such as scales sitting in the depressions between the ridges, aren't consistent with the relationship between scales and bone in living species, and there's no indication that other tissue types (e.g. cornified sheaths, armoured dermis) were present in these areas, so I think the ornamental ridge hypothesis is sensible (or, at least, not outrageously daft given the available data). I must admit to liking this hypothetical juxtaposition of fancy ornamental scales around the mouth and tough, reinforced tissues over the snout: perhaps tyrannosaurs weren't just big biting machines, but liked to look nice, too.

AMNH 5027 is just riddled with interesting surface textures that probably relate to epidermal features. To my reckoning, in addition to those depression/ridge pairings on the maxilla, the dorsal region of the lacrimal bears coarse projecting rugosity (armoured dermis); the top of the premaxilla and postorbital has a series of coarse hummocks (probable scales); and the ascending processes of the postorbital, lacrimal and maxilla are covered in a dense, anastomising network of neurovascular foramina (cornified sheath). What a neat looking animal Tyrannosaurus must've been - my take on this data is seen in the paintings accompanying this post. Image from Osborn (1912), in public domain.

Significantly, I can't find any tyrannosaurid skulls where these possible scale correlates extend right to the base of the maxilla (see photos, above). Rather, they terminate a few centimetres above the line of labial foramina, and this might have bearing on ongoing discussions about dinosaur lips. Scleroglossan lizards (the group that includes geckos, skinks, varanoids and amphisbaenians) frequently have osteoderms on their faces which cover their snouts (including the maxillae) except for a region around the labial foramina, which is smooth. This seems to relate to the presence of lip tissues displacing the scales from the skull and prohibiting formation of a epidermal correlate adjacent to the toothrow. Their maxillary juxtaposition of epidermal correlates is the same configuration that we see in tyrannosaurids as well as a number of other non-avian dinosaurs with maxillary epidermal correlates (e,g, pachycephalosaurids, ankylosaurids, some ceratopsids) and this has to be regarded consistent with hypotheses of extra-oral 'lips' in tyrannosaurids and other dinosaurs. If we add this to the evidence from foramina counts (Morhardt 2009, also see above) as well as other arguments for extra-oral tissues the case for crocodylian-like exposed teeth is looking increasingly doubtful. I must admit to thinking that proponents of exposed dinosaur teeth really need to start making better cases for this idea: most ways we can slice this particular debate suggests that extra-oral tissues are looking likely (and no, the common argument that their teeth were too big to be sheathed isn't valid: it's simply a speculation based on incredulity, not actual data from dinosaur skulls).

So...

To sum up this long, detail-heavy post:

My mural of a Diplodocus carnegii herd,currently keeping Dippy, the Natural History Museum's Diplodocus cast, company in Dorset County Museum. At 4 x 2 m, it's the third biggest picture I've ever done, and - as positioned at the museum - the most visible.

If you head to Dorset County Museum at some point before May 9th 2018 you'll be able to see a genuine dinosaur celebrity: the Natural History Museum's 'Dippy'Diplodocus skeleton, on the first stint of its 'Dippy On Tour' campaign of UK museums. The trip is well worth the visit even if you're familiar with the specimen from the NHM's Hintze Hall. A mezzanine around the skeleton, and the smaller size of the exhibition space, allows visitors to get closer to Dippy than ever before, and you can see the specimen from elevated positions unavailable at the NHM. If you're a sauropod fan in the UK, this might be your best chance to see this specimen up close and personal. It's free to see the skeleton, but you do need to book in advance - the tickets are flying off the shelves, so don't expect to just walk in.

A discerning audience checks out my prints at Naturally Curious. Say, some of those images look a little Life-through-the-Ages II-y...

Alongside Dippy is a collection of art entitled Naturally Curious, works by four different artists inspired by fossils and the natural world. My work is among them (above) and includes a 4 m wide mural based on the Dippy specimen and its palaeoenvironment - the same image that welcomed you to the post. It's not placed with the rest of my work but hanging right next to the Dippy skeleton itself - the first time a detailed artistic restoration has been associated with the specimen since the 1980s when a scale model stood next to its tail. This mural, commissioned by the Dorset County Council, was a great opportunity to bring Dippy's visitors up to speed on the latest ideas on sauropod dinosaur life appearance (as well as very flattering for me - it's not every day you're asked to display art next to one of the most famous dinosaurs in the world). The process involved learning a lot about the Dippy specimen, applying some new ideas about dinosaur anatomy to Diplodocus, and looking into the specifics of Dippy's palaeoenvironment. If that's not fodder for a blog post, I don't know what is.

Because production time on the mural was short, we decided to augment an existing picture rather than start from scratch. The image in question is below, and was created in 2009 to publicise work by Mike Taylor, Mathew Wedel and Darren Naish on sauropod neck posture (Taylor et al. 2009). The Dorset team liked the image and, though quite dated now, it gave an anatomical and compositional framework that had been approved by several sauropod experts, shaving a lot of time off the production schedule. The final artwork is different to the original in many respects but much of the 2009 DNA remains obvious, including our nod to Rudolph Zallinger's Age of Reptiles mural.

PR art for Taylor et al. (2009), showing D. carnegii with its neck held aloft rather than - as was fashionable at the time - held horizontally. 2009 was a long time ago for me, artistically speaking.

Working with Dippy, and establishing the scene

It's important to any palaeoartwork to know the nature of the actual fossil material behind a reconstruction, and it might come as a surprise to know that 'Dippy the NHM Diplodocus'* is a different entity to the specimen it's cast from. The 'real' Dippy is Carnegie Museum specimen 84 (CM 84 for short), the holotype of Diplodocus carnegii, unearthed from Jurassic sediments of Wyoming in 1899. It's a mostly-complete skeleton missing elements of the limbs, the end of the tail and the skull, and these elements were sculpted or casted from other animals to create the mounted Dippy skeletons in museums around the world. This makes Dippy mostly representative of a single individual, but still a composite of several Diplodocus. CM 84 has been extensively documented - especially in Hatcher's 1901 monograph - and this makes it an excellent specimen to base a palaeoartwork on. Scott 'Master of Dinosaur Bones' Hartman's 2013 Diplodocus skeletal restoration was used to fill in the proportional gaps, and Tschopp et. al (2015) provided some very useful data on diplodocid osteology, often up close and in clear detail.

*The NHM's CM 84 cast is not the only Diplodocus to bear this nickname: the actual CM 84 specimen was also christened 'Dippy' when discovered in 1899, and several museums around the world use this name for their casts. In this article, my use of 'Dippy' consistently refers to the NHM cast.

Where the Diplodocus roam: depositional settings of the Morrison Formation at the time when Dippy lived. The Dippy site itself is in southeast Wyoming, among the series of wetlands that line the eastern side of the Morrison basin. From Turner and Peterson (2004).

CM 84 stems from the centre of the Morrison Formation, a famous Late Jurassic unit that yields, in addition to Diplodocus, many famous dinosaurs: Allosaurus, Stegosaurus, Brontosaurus, Ceratosaurus and Camarasaurus, among others. The Morrison Formation is geographically extensive with major outcrops in Colorado and Wyoming, and additional exposures in 11 other states (above). Palaeoenvironmental studies show variation in climate and habitats across that range. We know that southern regions were drier, that a number of water bodies existed across the basin, and that water and sediment influxes were received from highlands to the west and, possibly, the east (Turner and Peterson 2004). The Wyoming quarry where CM 84 was recovered represents an ancient lake, part of a broader series of wetlands in the east of the Morrison depositional basin (Turner and Peterson 2004; Brezinski and Kollar 2008). It's been suggested that these relatively well-watered settings may have been important habitats for dinosaurs of all kinds, offering abundant plant material compared to the surrounding arid environments (Turner and Peterson 2004). I took these details on board for the mural, changing the backdrop of the 2009 image from a sparse lake margin to a well-vegetated, westward-facing gateway with distant hills. The result is hopefully something not too far off the environment that CM 84 was buried in, and maybe lived in.

Proportions, poses and pedes

Although some tweaks were made to the proportions of the animals from my 2009 image, the basic poses of each was maintained. Readers may question why the necks of the animals have remained aloft when some researchers and artists still use the horizontal neck poses popularised in the late 1990s. The primary basis for horizontal sauropod necks are the famous Dinomorph digital models (Stevens and Parish 1999 and subsequent works) and, though debates on these matters continue, a number of papers have found issues with these models, to the extent that I'm not sure they're reliable at present. Rather than summarise these issues here, I suggest you simply read Mike Taylor and Matt Wedel's blog series on sauropod neck posture over at SV:POW!- all the citations and discussion you need are there, and in much greater detail than I could cram into one paragraph.

This means that the postures used in my mural are still, almost 10 years on, based on the conclusions of Taylor et al. (2009). If you missed this paper (which you need miss no more, seeing as it's open access), it used x-ray data to show that all extant terrestrial amniotes habitually hold their necks with an elevated base during idle but alert behaviour. As summarised by Mike and colleagues in their abstract:

"Unless sauropods behaved differently from all extant amniote groups, they must have habitually held their necks extended and their heads flexed."

Of course, no-one is saying that sauropods could only carry their necks aloft: we're talking about their default, habitual pose, not those employed during other behaviours like foraging or drinking. Here's artistic proof.

My PR art for the description of the titanosaur Shingopana songwensis, with another titanosaur - Rukwatitan bisepultus - in the distance. Notice the bulk in their torsos - the chests of diplodocids were a world away from these chunkers.

A very Dippy face-lift

Alamosaurus, Dakotaraptor and a collection of tourists approach TimeBase 67. Note that the necks of Alamosaurus are not hugely oversized, but augmented with a long skin flap along the underside.

These Dakotaraptor don't look too pleased with this Alamosaurus on their patch! He doesn't seem too bothered though...

Book now to see these living dinosaurs up close: https://t.co/HMjoxajkBepic.twitter.com/XIFog4AQ9N

— DinosInTheWild (@DinosInTheWild) November 17, 2017

It's not all yawning dinosaurs and preening pterosaurs, though: fans wanting dicier threat displays and hunting behaviours won't be disappointed**. Happily, the quality of the reconstructions matches the depicted behaviours. The animals are thoroughly modern takes on familiar species and seem to have received refreshingly little, if any, embellishment to make them more ferocious or marketable. Extra-oral tissues (lips and expanded rictal plates) are standard, bold but credible decisions have been taken with their integument, and the volume of muscle and other soft-tissues is substantial, but within reason. Their animations are pretty good too, with larger species having an appreciable sense of mass and inertia instead of pirouetting around like creatures half their size. This is especially noticeable when the animals are close to viewing windows, these being large enough to appreciate their real-life size. A lousy sense of mass in the animation would have ruined the illusion, but they move with a weight and heft comparable to large living animals. This might not be something that we appreciate consciously, but is one of narrowest precipices over the uncanny valley and the downfall of many dinosaur animations. Hats off to the animators for taking time to get it right.

**Apparently. I, er, was basically watching the pterosaurs most of the time.

Variation in animal proportions, integument and colouration gives a sense of looking at real populations and not cloned digital models - it's subtle, but makes all the difference. I suspect deliberate efforts were made to avoid the uniform greys and browns that still characterises many popular dinosaur reconstructions with most species sporting elaborate colouration or patterning somewhere. These are not garish carnival monsters though, and look consistent with our knowledge of pigment mechanics and evolution, as well as appropriate for the habitat and lifestyle of the creatures concerned. Ultraviolet colouration and iridescence features too. Further points are awarded for the animals not being dressed up in the colours of living species: there's no cassowary-inspired maniraptorans or other obvious real-world colour schemes to jar the illusion. I'm fairly certain the facial colours of Triceratops (below)owe something to Darth Maul, though...

DITW's Triceratops takes a dip. Sadly, this great image isn't a still from DITW proper, but the attention to detail and nuanced behaviour shown here - including the birds on the Triceratops face - is typical of the show in general.

Several aspects of the reconstructions recall All Yesterdaysfor their boldness, such as the pterosaur dewlaps, display flaps on the sauropod necks, and some inflatable nasal tissues - this isn't surprising when you realise that an All Yesterdays author - Darren Naish - was DITW's scientific consultant. I'm sure these additions will startle some folks who aren't familiar with modern palaeoart conventions, especially those used to dinosaurs depicted as shrink-wrapped walking musculoskeletal systems, but, simultaneously, none of the animals look 'over speculated': their appearance acknowledges our limits to predict extinct animal anatomy without losing sight of what real animals look like. This isn't to say there aren't some aspects of the reconstructions that won't be quibbled by experts, but we're talking nitpicks here, not glaring problems. In terms of broad-brush strokes, and most of the finer ones, DITW hits home in all critical aspects of its reconstructions and animations. It's rare to see big-budget, mass-audience palaeoart achieve this sort of credibility and is especially surprising given how much animation was needed to create the sense being in a real 3D environment, sometimes with multiple views over the same landscape and animals transitioning between viewing stations. The sensation is believable enough that, upon leaving the event, I felt a strong urge to head off to the countryside for genuine wildlife watching.

Back in the real world, away from the TimeBase, is where some minor criticism of DITW might be found. Having been thoroughly impressed with DITW I was a little disappointed to find that there was no book or other media (behind the scenes DVD? Blu-ray with the animations?) allowing us to preserve the experience at home. There's some great work gone into this show and it's a shame to think that, when DITW eventually ends its run, there'll be no way to truly appreciate the designs and ingenuity that went into it. There are DITW toys, posters and clothes, but they only go part way to capturing the experience itself. A book or 'behind the scenes' film could reinforce the science behind the spectacle, too - a lot of visitors surely enjoy DITW, but do they know how saturated in science the event is? I'm aware that this might change in the future - I hope it does.

Another look at the DITW Tyrannosaurus. There are other, non-tyrannical choices of PR art, but I really like the composition of this piece. Half-obscured dinosaurs have an almost classical vibe, I suspect Zdenek Burian would have approved. The artworks you're seeing in this post are promotional renderings by Damir Martin - check out his site for more cool stuff.

The ticket cost of DITW has drawn some comment on social media, some of which may be unwarranted. Tickets are upwards of £20 each so, yes, DITW is undeniably a more expensive dinosaur experience than, say, visiting a museum or watching the next Jurassic World movie at the cinema. Such cost, however, is comparable to that of gigs, theatre shows and travelling exhibitions - not perfect analogues for DITW, but similar in terms of running expenditure and event duration. I'd argue that the novelty, ambition and execution of DITW trumps most of these experiences too: there really isn't much else out there like it, let alone something of such quality and educational potential. I appreciate this doesn't diminish what will be steep door prices for some, but a little online research revealed a number of family passes, promotional codes and other means of trimming the ticket cost down, sometimes quite considerably. Group rates are available too, if you're looking to attend with a suitably sized clan. If cost is an issue I heartily recommend checking these offers out - dinosaur fans young and old(er) will not want to miss this.

In sum, Dinosaurs in the Wild is a terrific blend of cutting edge science, technology and entertainment that dinosaurophiles - or anyone with a general interest in extinct life - will enjoy immensely. Whether you visit just for the spectacle, to nerd-out over the palaeontological Easter eggs, or to see what next-generation science outreach could be like, you're sure to enjoy it. As far as I know, the next location for DITW has not been announced yet. It leaves London on the 31st of July, so I recommend exploiting London's accessibility to visit as soon you can in case the next venue is less reachable. Hopefully, if DITW does well, we can look forward to sequel experiences set in alternative times or locations: DITW-style experiences in Mesozoic seas, Permian Russia or Pleistocene Europe, anyone?

Exclusive, unused promotional still of the DITW Alamosaurus, AKA Spods Maclean, finished acting for the day and out of character, relaxing in a pub close to the venue. It's funny how performers are often smaller in life than they appear onscreen.

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Quick painting of Polyphemus, the Homeric cyclops, taking very literal inspiration from elephant face anatomy in reference to the well-known idea that fossil elephant skulls inspired the cyclops myth. So, do ancient illustrations of cyclopes count as early palaeoart?

The genre of natural history art we call 'palaeoart' is not a modern invention: it is actually centuries old, emerging in Europe at the same time as palaeontological science. We often credit Henry De la Beche's 1830 painting Duria Antiquior as the original palaeoartwork, but several attempts to reconstruct fossil animals using modern scientific ideas were made beforehand, dating back to at least 1800 (Taquet and Padian 2004). They include relatively speculative paintings, satirical sketches, and detailed anatomical reconstructions (Rudwick 1992; Martill 2014). Duria Antiquior was a major milestone for palaeoart development, but not the origin of the genre itself.

A case can be made for palaeoart being even older than these oft-overlooked works, however. A small number of artworks created by historic, maybe even ancient peoples attempted to restore the life appearance of fossil animals in much the same way we do today, albeit outside of a true scientific context. Whether or not these artworks qualify as true palaeoart is questionable as adherence to scientific theory is a pretty major component of the genre. Scientific methodology as we understand it today was not developed until the 18th century, and this included many concepts essential to palaeoart, such as fossilisation, extinction and geological time. Can we truly define a work as palaeoart if it was made without knowledge of these cornerstones of palaeontological science? My take on this is that artworks attempting to rationalise fossils against contemporary understanding of natural phenomena (even if that rationale is pre-scientific and mythology-based) have the same intention as palaeoart produced today. We can probably consider these early artworks 'proto-palaeoart', the forerunner of the genuine, science-led article we developed in the 19th century.

I thought it might be of interest to run through some early artworks claimed to be among the oldest palaeoart. I won't pretend that this list is exhaustive, but I hope there may be some examples, or facts behind commonly given examples, that will be unfamiliar to most readers. In researching this article, I was surprised at how little data existed behind some claimed examples of historic palaeoart, including several widely 'known' examples. Other cases are more plausible, if missing smoking gun evidence, and a couple are undoubted facts of history. For those interested in the origins of palaeoart, the question is not 'does proto-palaeoart exist?', but 'how much proto-palaeoart is there?'

Of griffins and cyclopes

Archaeological data shows that humans have been interacting with fossils for thousands of years (McMenamin 2007; Mayor 2011). It is not unreasonable to assume that ancient peoples pondered the nature of fossils and perhaps drew or sculpted the creatures they were interpreted as. Othenio Abel (1914) and Geraldine Mayor (2011) have argued that fossil remains influenced or even wholly inspired famous mythical animals such as griffins and cyclopes. As previously discussed here at some length, some researchers propose that fossils of the Asian horned dinosaur Protceratops were subsumed into the mythology of the griffin (e.g. Mayor and Heany 1993; Mayor 2011), while the bones of elephantids – with their huge, eye-like central nasal openings in their skulls – spawned stories and artwork of the one-eyed cyclops (Abel 1914).

Line drawing of perhaps the oldest known image of a griffin, from Susa, 4th millennium BCE. From Frankfort (1937).

Superficially, both these claims seem reasonable. Griffins, if you squint a little, do somewhat resemble a Protoceratops with their four legs, beaks and cranial frills interpreted as wings. The skulls of elephants and their relatives look somewhat like the skulls of monstrous giant humans, too, mostly because of their short faces and partially-defined true eye sockets. But what's lacking from these claims is evidence beyond the circumstantial. The Protoceratops-griffin hypothesis is presented as having support from historic events, geographic details and ancient texts, with traders from far eastern lands bringing tales of their fossils to the Greeks in the first millennium BCE. Long term readers may remember I suggested a number of issues with this scenario in a previous article. I won't rehash the full argument here but, in brief: griffins appeared in Near East societies several millennia before they became popular in Ancient Greece, meaning the Orientalisation of Greece during the 8th-5th centuries BCE - when the Greeks adopted culture from Near Eastern and Eastern Mediterranean cultures - more than accounts for the sudden Grecian interest in griffins. Ancient texts said to refer to Protoceratops fossils seem to refer to (probably fantastical) living species, not fossils, and provide no details of geography of environment that are specific to genuine Protoceratops localities. The trade routes and gold mines said to bring Asian cultures within viewing distance of Protoceratops remains are, in fact, several hundred miles west of all known Protoceratops sites, and there's nothing about griffin form - in any of its guises (griffins are a complex of creatures, not just one) - that necessitates influence from horned dinosaur anatomy: all griffin features are accounted for by living species. Citations and references for these points can be found in my article, so please check it out if you'd like more details. I've not encountered anything since writing that piece to change my opinion on the Protoceratops-griffin hypothesis, so I can't see any reason to consider griffins proto-palaeoart of horned dinosaurs.

Historic and biogeographic details align better with the idea that elephantid fossils may have begat cyclopes. Fossils of elephantids are found around the eastern Mediterranean and their bones were probably known to the Ancient Greeks (Massetti 2008; Mayor 2011). It's plausible that Greeks living several thousand years BCE would be ignorant of living elephants too, these animals dying out in Europe around 11,000 years ago. The nearest contemporary elephant populations were of the now extinct Syrian elephant, over 1000 km away in eastern Turkey. Elephant skulls are pretty odd, and without knowledge of living elephants it might be easy to misinterpret them. Homeric accounts of cyclopes - from the 7th-8th century BCE, among the earliest on record - cast them as cave dwellers, which matches the recovery of elephantid remains from Sicilian caves (Masseti 2008). The link between these bones and cyclopes has been noted for centuries, dating back to the first 'modern' archaeological exploration of Mediterranean islands in the 17th century (Masseti 2008).

A funerary urn showing the cyclops Polyphemus being blinded by Odysseus and his crew, c. 660 BCE. From Wikimedia user Napoleon Vier, CC BY-SA 3.0

These details put elephantid bones in the right place and time to inspire cyclopes but, as hardened sceptics, we must view this as circumstantial evidence only, and thus insufficient to support the elephantid-cyclopes link on its own. It's here where we hit a problem: beyond these details, there's not much else to support this idea. It's important to ask the right questions in sceptical inquiry and in this case it's not 'did elephants inspire cyclopes?, but 'do we need elephants to explain depictions of one-eyed giant humans?'. The answer is probably 'no'. Accounts of ancient cyclopes I'm aware of - both those in illustration and literature - are just giant men with unusual eye anatomy (example above), and without obvious elephantine facial features (tusks, steep-fronted rostra etc.), citing elephant skulls as a source might complicate the myth more than explaining it - where's the rest of the elephant anatomy gone? An entirely human source - cyclopia, a fatal genetic condition sees human eye anatomy fail to divide fully - is an alternative source of the cyclopean myth (Kalantzis et al. 2013) which does not require artists to cherry-pick elephant features. Cyclopia is rare among live human births (Kalantzis et al. 2013) but occurs in one of every 200 lost pregnancies - as sure as ancient Greeks saw fossil elephant bones, they also surely saw patients of cyclopia.

We must also consider that a real-world source was not needed at all. One-eyed men and other monocular creatures are ubiquitous throughout mythology all over the world, and it's unlikely they all developed after finding fossil elephant skulls. Eyes are a well established symbol of wisdom, clairvoyance and authority in many cultures, so the modification of eyes - reduction in number, blinding and so on - has clear symbolic value in many legends. It's entirely plausible that Grecian cyclopes had one eye simply because the ancient poets and storytellers thought it suited their characters. It's difficult to prove that fossil elephant skulls were not the basis for cyclopes but with only circumstantial evidence to support the idea, it's no better supported than any other interpretation outlined here or elsewhere.

The Monster of Troy

An artwork argued by Mayor (2011) as the oldest piece of genuine palaeoart adorns a Corinthian vase painted between 560-540 BCE. This image shows an unusual, skull-like face resting on a cliff acting as the Monster of Troy, the creature which fought Heracles as it terrorised Hesione at the outskirts of Troy. Though skeletal in nature, the interactions of the face with other figures on the vase implicates it as a living creature, not the remains of a dead animal. The skull is argued to match the basic anatomy of Miocene mammals known from the eastern Mediterranean region. The giraffid Samotherium is considered a most likely identity (Mayor 2000, 2011), though the artist may have also incorporated elements of fossil ostriches, lizards, whales or crocodiles (Mayor 2000, 2011). If this hypothesis is correct, it would easily be the oldest known palaeoart, and by a huge margin - about 2000 years. Mayor's interpretation has been discussed favourably by a number of authors (Papadopoulos and Ruscillo 2002; McMenamin 2007), though others consider it a matter of ongoing research (Oakley 2009) or pure conjecture (Kitchell 2014).

The Monster of Troy, as depicted on a Corynthian vase, 560-540 BCE. It definitely has a skull-like vibe, but is it the first piece of palaeoart? From Flickr user Lady Erin, CC BY-NC-ND 2.0.

The individualistic nature of the Monster of Troy complicates analysis of its origin, especially because it seems quite loosely drawn compared to other figures on the vase. How literally should we take its features? If we had other, perhaps more refined art of the same concept we might be able to pin down the accuracy of its rendition but, with only one example, we can't be sure if we're dealing with a crude drawing of a real skull or a more stylised, imaginative chimera.

If we take the face entirely literally, we find that some aspects compare well to mammals like Samotherium, particularly the size, the shape of the lower-jaw, the position of the jaw joint with respect to the orbit, and the low profile of the rostrum. However, it differs from Samotherium in a number of ways: a lack of horns; entirely procumbent dentition; long, sharp-looking teeth; a lack of a diastem; the (seeming) presence of a sclerotic ring; and the occurrence of a facial fossa (present in fossil horses and deer, but not Samotherium). The white colour is also not appropriate for Samotherium, fossils of these animals being of tan or brown hues. Some distinctions are potentially explainable within the Samotherium hypothesis: the shortened upper jaw could reflect a broken premaxilla - a common occurrence on large fossil mammal skulls - and the unusual detailing behind the eye could reflect details of the jaw joint and posterior skull anatomy. Others differences are less easily accounted for, leading to those suggestions that lizards, whales and other species might be referenced in the illustration too. This seems like special pleading to me, and a weakness in the idea that the artist was referencing specific fossil specimens. The only evidence for the Monster of Troy being a fossil is that it allegedly looks like one. If we find differences between it and the fossils it's mostly likely to represent, they can't just be glossed over, they're counter-evidence to the hypothesis.

Samotherium boissieri skull - is this the 'real' Monster of Troy? By Wikimedia user Ghedoghedo, CC BY-SA 3.0.

Again, I wonder if we need to invoke fossils to explain this illustration. The basic anatomy might reflect some features of ungulate skulls, but it's so generalised that something like a living horse or camel would fit the bill as well as a fossil species. Indeed, some aspects - such as colour - are better matches for modern skulls. The fact it's perched on a cliff is perhaps the best reason to think it's a fossil, though other interpretations of the 'cliff' exist, such as it being the entrance to a cave (see Mayor 2000 for a brief summary of other interpretations).

All this considered, I'm not sure what to make of the Monster of Troy. I'm not convinced it's a compelling match to a specific fossil mammal skull nor that it even needs a fossil origin to explain it. Moreover, if it is a chimera, which even proponents concede it must be to some extent, then its significance to early palaeontology is diluted further as those other elements may not be of fossiliferous origin. If we had other illustrations of the same skull-like creature we might be able to make a clearer determination, but I don't know that there's enough evidence from this image alone to determine if the Monster of Troy is anything to do with the history of palaeoart.

Here be Lindwurms

Moving on two thousand years to the 16th century, our next example is an artwork with a confirmed fossil basis. Our inquiries into artwork from this time onward are aided significantly by surviving texts from this interval. As we've already encountered, interpreting the origin of art is challenging without knowing the context of its creation, so the existence of well-documented artefacts and text allows for much more certainty in our pursuit of pre-science palaeoart. Much of the following stems from Abel's (1939) account of fossils and mythology.

The giant Lindwurm statue of Klagenfurt, Austria, built in 1590. It's said to be partly informed by woolly rhinoceros remains. The chap on the right, representing Hercules, was added in the 17th century. From Wikimedia user Johann Jaritz, CC BY-SA 3.0.

Though 16th century Europe heralded many major facets of our modern age, myth and fable were still major parts of culture, and giant fossils were still regarded as remains of fantastical animals. A vast, 6 tonne statue of a four-limbed, two-winged dragon known as the ‘Lindwurm’ is probably the oldest known incontrovertible piece of proto-palaeoart. Only part of the statue, which was erected in Klagenfurt, Austria in 1590, has a fossil basis however, its head being based on the skull of a woolly rhinoceros (Coelodonta antiquitatis) recovered from a gravel pit or mine near Klagenfurt in 1335. The Lindwurm has a prominent role in Klagenfurt lore as the town was said to be founded only after this creature was dispatched and the area became safe to live in. I'm not sure if the skull or the legend came first - the town was established in the 12th century, two centuries before the skull would be found - but we can be certain that Coelodonta fossils have longstanding historical significance in Klagenfurt, the skull residing in town council chambers for centuries before being put on public display, where it remains today. The statue was constructed by Ulrich Vogelsang, but it's evident that he only considered very basic elements of Coelodonta anatomy during the sculpting. Indeed, other than size, the Lindwurm head does not resemble Coelodonta at all, so it seems likely that the skull was more inspirational than referential. Still, at least we know the two objects were meant to represent the same entity, which is no mean feat in the pursuit of proto-palaeoart.

The giants and plesio-dragons of Mundus Subterraneus

Athanasius Kircher's 1678 German textbook Mundus Subterraneus - an early thesis on geography, biology, mineralogy and geology - contains several illustrations of animals which may have been informed by fossils. They include many types of giant human, which were said to be social, cave-dwelling species based on the bones of large animals found in caves - almost certainly remnants of Pleistocene mammals. Kircher also wrote about several types of dragon, many of which were of period-typical, worm-like form, but Abel (1939) noted one unusual dragon illustration that may have been influenced by a real giant reptile: a plesiosaur.

Is St. George fighting a plesiosaur-inspired dragon in this 1678 illustration from Mundus Subterraneus? Abel (1939, also the source of this image) thought so, noting the shift towards plesiosaur-like proportions and anatomy compared to more conventional European dragon depictions of the time.

The illustration is plesiosaur-like in many respects, with a barrel-like body, small head, long and slender neck, a true tail, and curiously small ‘paddle-like’ wings instead of broad, membranous wings typical of dragon depictions. It's not a perfect plesiosaur depiction by any means - it also has ears, a beak, and four legs - but Abel (1939) considered this reinvention of dragon form so dramatic that it could represent the arrival of a new source of inspiration for dragon anatomy, of which plesiosaurs are a possible contender. Marine reptiles, including plesiosaurs, were almost certainly uncovered during quarrying work in the historic Swabia region (now southern Germany) as rocks we now call the Posidonia Shale were exploited to build growing settlements. The Posidonia Shale is a site of exceptional preservation with abundant invertebrate fossils and rarer, but often complete and articulated, marine reptile skeletons. Posidonia quarrying dates back to at least the 16th century and, given that the quarrying was executed by hand, 17th century quarrymen would have seen fossils of many kinds, almost certainly including some well preserved plesiosaur remains. Had these discoveries caused a stir among local learned individuals, as well a giant reptile entombed in stone might have, it's not inconceivable to think they could have been identified as dragons, and ultimately influenced Mundus Subterraneus.

As with our discussion of cyclops art, these details are only circumstantial evidence and they do not prove beyond doubt that plesiosaurs were referenced in Kircher's dragon art. But I find this case a little more compelling because our records of the early modern period are better, so the correlation between historic events is tighter and the contrast to other dragon illustrations more obvious. Moreover, whereas ancient cyclops art doesn't really look like the fossils said to inspire it, I can see some obvious plesiosaur-like details in Kircher's illustration. It's difficult to be certain about the relevance of plesiosaurs fossils to the image but, for me, this is a possible, if unconfirmed, piece of proto-palaeoart.

The most awesome unicorn, ever

Our final example is surely one of the nuttiest attempts to restore ancient animal anatomy in all of history. Pleistocene mammoth and rhinoceros bones found in a cave near Quedlinberg, Germany, in 1663 were reassembled by an unknown artist into a skeletal reconstruction of a bipedal unicorn, christened unicornum verum ('true unicorn') or, sometimes, the Quedlinberg Monster. Doubtless this image is familiar to many readers already, but it's worth looking at again. Just how is that thing meant to work?

Page from the 1749 book Prototagea showing unicornum verum, a truly bizarre composite of fossil rhinoceros and mammoth bones. The illustration above is clearly a mammoth molar, hinting at the true identity of the 'unicorn' bones.

The artistic history behind unicornum verum is somewhat mysterious (Ariew 1998). The illustration became widely known through Gottfried Wilhelm Leibniz's posthumously published 1749 book Protogaea, a scholarly account of geology and natural history. Leibniz's book printed a copy of one example of the illustration, but did not state where the images originated. The most famous example - above - is often credited to German naturalist Otto von Geuricke, the scholar who described the remains, or Leibniz himself. However, Geuricke was probably not the artist, and Leibniz definitely wasn’t (he explicitly states this in his written work). Another version of the skeletal, published in 1704, is said to be based on a third depiction by Johann Mäyern, a Quedlinberg counsellor. Whoever rendered the images, they represent the oldest known illustrations of restored fossil skeletons (we might quibble if skeletal reconstructions are true palaeoart or not - whatever your view, they're close enough for our purposes here, I think). Though some bones are fairly 'generic' and difficult to identify, mammoth teeth and scapulae, as well as rhino vertebrae with long neural spines (reversed to be ribs) are discernible. I am not sure what the ring-shaped structure at the end of the spine is - I assume it's a vestigial pelvis. Apparently the bones informing the skeletal were broken as they were excavated (Ariew 1998), which might account for some peculiarities of their appearance.

Unicornum verum in the flesh. It's a little undersized: Leibniz gave the length of the horn at five ells (an ell being the length of a man's forearm (typically about 450 mm, or 18"), which is over 2 m.

Leibniz indicates that narwharls were a major influence on unicorn mythology of this time, which might explain why unicornum verum resembles a swimming animal to some degree. The reconstruction is so unusual that some scholars have wondered if it was a joke or hoax. Ariew (1998) suggests Leibniz - a polymath of notable contribution to mathematics, physics, philosophy and other fields - was an unlikely hoaxer based on his other work. Indeed, Protogaea is by all accounts a straight, scholarly thesis on natural history which demythologises fossils and calls out fantastic interpretations - trickery and pranks would contrast markedly with the tone of the book. Leibniz also says he visited the caves housing the bones in question, providing details of how one enters them, and vouched for the size, manner of collection and anatomical details of the bones found therein. If he was hoaxing, he played a very straight game, and it's perhaps more probable that he considered unicornum verum a genuine animal, and the illustrations a reasonable take on its anatomy.

By the end of the 18th century the seeds of true palaeontological science and palaeoart were being sowed, ready to develop fully in the 19th century. Leibniz's apparent conviction for unicornun verom and its illustration might seem charmingly naive given what would emerge just decades after Protogaea was published, one of the last examples of mythology inspiring scientific thought and early palaeoart before hard science took over. But his illustration of a restored skeleton, rather than a fanciful creature, as well as his associated documentation of the discovery and locality of the 'unicorn' bones, shows how approaches to fossils and their illustration was maturing. This bizarre restoration is a link between two different eras in our artistic interpretations of fossils, taking a near-scientific approach to a mythological concept.

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References

• Abel, O. (1914) Die Tiere der Vorwelt, Leipzig-Berlin, B.G. Teubner.
• Abel, O. (1939). Vorzeitliche Tierreste im Deutschen Mythus, Brauchtum und Volksglauben. Jena (Gustav Fischer).
• Ariew, R. (1998). Leibniz on the unicorn and various other curiosities. Early Science and Medicine, 3, 267-288.
• Kalantzis, G. C., Tsiamis, C. B., & Poulakou-Rebelakou, E. L. (2013). Cyclopia: from Greek antiquity to medical genetics. Italian Journal of Anatomy and Embryology, 118(3), 256.
• Kitchell Jr, K. F. (2014). Animals in the Ancient World from A to Z. Routledge.
• Masseti, M. (2008). The most ancient explorations of the Mediterranean. Proceedings of the California Academy of Sciences, 59(1), 1-18.
• Mayor, A. (2000). The ‘Monster of Troy’Vase: The Earliest Artistic Record of a Vertbrate Fossil Discovery?. Oxford journal of archaeology, 19(1), 57-63.
• Mayor, A. (2011). The first fossil hunters: dinosaurs, mammoths, and myth in Greek and Roman times. Princeton University Press.
• Mayor, A., & Heaney, M. (1993). Griffins and Arimaspeans. Folklore, 104, 40-66.
• Martill, D. M. (2014). Dimorphodon and the Reverend George Howman's noctivagous flying dragon: the earliest restoration of a pterosaur in its natural habitat. Proceedings of the Geologists' Association, 125(1), 120-130.
• McMenamin, M. A. (2007). Ammonite fossil portrayed on an ancient Greek countermarked coin. antiquity, 81(314), 944.
• Oakley, J. H. (2009). Greek vase painting. American Journal of Archaeology, 599-627.
• Papadopoulos, J. K., & Ruscillo, D. (2002). A Ketos in early Athens: an archaeology of whales and sea monsters in the Greek World. American Journal of Archaeology, 187-227.
• Rudwick, M. J. (1992). Scenes from deep time: early pictorial representations of the prehistoric world. University of Chicago Press.
• Taquet, P., & Padian, K. (2004). The earliest known restoration of a pterosaur and the philosophical origins of Cuvier’s Ossemens Fossiles. Comptes Rendus Palevol, 3(2), 157-175.

Giant azhdarchid pterosaur in flight. Images like this virtually always trigger discussions about the validity of giant pterosaur flight hypotheses.

With the 25th anniversary of Jurassic Park cascading through dinosaur social media you could be forgiven for overlooking another influential dinosaur franchise celebrating the same vintage this year. Unveiled in 1993, this long-running series stood out for showing dinosaurs as fast, agile and intelligent animals, and immersed us in an then-unparalleled expanded prehistoric narrative, rich in detail and huge in scale. I'm talking, of course, about Ricardo Delgado's Dark Horse comic series Age of Reptiles.

The award-winning Age of Reptiles series comprises multiple, unconnected stories published throughout the last 25 years, with a bit of a hiatus between its earliest and latest incarnations. The series comprises four serials (Tribal Warfare, 1993; The Hunt, 1994; The Journey, 2009; and Ancient Egyptians, 2015) and two shorter pieces (The Body, 2011; Baby Turtles, 2014 - regrettably, I haven't seen these in entirety). It has a number of fans among those of us who research and illustrate fossil reptiles, and judging from the calibre of movie makers who've contributed endorsements to the comics, it is well regarded in the movie industry, especially for its entirely 'silent', visual means of storytelling. Despite some relatively complex narratives, large casts and use of multiple locations, not a word of dialogue or descriptive text is used to explain plots or character motivations. The 2011 documentary Dinosaur Revolution and its 2012 spin-off Dinotasia were inspired by Age of Reptiles, with Delgado having director credits on two episodes of the former. It's fairly well known that, until late in production, Dinosaur Revolution was effectively meant to be Age of Reptiles: the TV show, but studio cold feet revised the programme into a more conventional documentary.

Cover art for various editions of the Ricardo Delgado's Age of Reptiles comic series, borrowed from Dark Horse Comics. Their website which has a full back catalogue of the series for your browsing and purchasing needs. Sorry for the low res images - I'm deliberately using officially released artwork in this post to avoid unintentional piracy of Age of Reptiles content. Entire sequences of the Age of Reptiles comics have been uploaded without authorisation to the web, and that's not cool, folks: it's stealing.

Age of Reptiles was an important influence on my childhood drawing and, with the series hitting the big two-five this year, I thought I'd share some of what I think makes the series special. It helps, I think, to set the stage in which I first met Age of Reptiles as an eight or nine-year old* dinosaur obsessive in the early 1990s. Though not so long ago, this was a different age for palaeontological media because rendering life-like prehistoric animals for TV or film was much harder than it is today. Making a realistic film or documentary chronicling the lives of prehistoric animals would not only have been very difficult, but also very expensive. Jurassic Park may have broken new ground for dinosaur animation in 1993, but it needed sophisticated animatronics, then-radical computer generated imagery and a Hollywood-grade budget to achieve its visuals. It took several years for this technology to become more widely affordable, with the the bridge between Jurassic Park and our living room viewing - Walking with Dinosaurs - not appearing until 1999. Thus, if I wanted to see 'living' dinosaurs, rather than just disconnected pictures in dinosaur books, I had to made do with the short vignettes with wobbly puppets on the A&E Dinosaur! series, or else hope to find a showing of a Harryhausen dinosaur film on TV.

*My birthday would have been around the time I first saw the comic, and I can't remember if I'd graduated to nine years old when I first saw it.

It was into this landscape that Age of Reptiles placed a weighty clawed foot. I first encountered the first story, Tribal Warfare, through the UK's take on the Jurassic Park comics. Coinciding with the movie release, the international branch of Dark Horse comics published a weekly Jurassic Park magazine that contained a comic story of the movie as well as two other series: Xenozoic Tales (a post-apocalyptic sci-fi story with minimal dinosaurs - I never really got on with it) and something called Age of Reptiles. Juxtaposed against the talky, high-tech worlds of Jurassic Park and Xenozoic Tales, Tribal Warfare immediately stood apart with its silent, patient and entirely immersive opening. Without a word of introduction, we see a sleepy Pteranodon wake up, spread its wings, and then launch over a huge, double-page vista of unspoilt trees and bluffs. The pterosaur sails past a foraging sauropod, which we soon learn is being stalked by a group of dromaeosaurs. We watch the sauropod flee and ultimately fall to its attackers, before a huge tyrannosaurid arrives to claim the carcass from the smaller predators. The animals were colourful, dynamic, imposing and vicious, and played out their drama without interruption from a narrator, talking head or visiting caveman. It was unlike anything I'd ever seen.

Sorry, Walter Cronkite: your A&E dinosaur puppets had nothing on this. Page from the opening comic of Age of Reptiles: Tribal Warfare, where a pack of Deinonychus bring down a titanosaur, while Pteranodon soars past. Anachronistic fauna be damned: for an early 1990s dinosaur fan, this was ambrosia from the loftiest peaks of Olympus. Borrowed from Dark Horse Comics.

Reading the comic in entirety, it was apparent that this was not a story about prehistoric animals living with humans, or anthropomorphic cuddly dinosaurs learning lessons about friendship. Age of Reptiles was the extended, unadulterated prehistoric drama every '80s kid wanted but film or TV had yet to produce. OK, it was in comic format rather than animated on screen, but Delgado's experience with storyboarding and film illustration gave our brains little work to do as we filled in the action between panels.

Age of Reptiles continued to resonate for years after I first encountered it. Much of the dinosaur art I drew for the next 5, 10... 25 years was influenced to a greater or lesser extent by Delgado's creation, to the extent that I rank him as one of my top artistic influences. It may not be as obvious in my modern work as it was 20 years ago, when I was a teenager liberally borrowing from his style (below), but it's still there. Every now and then a Delgadoesque waterfall or critter still sneaks into one of my paintings and I still have a lesson in composition and visual storytelling whenever I re-read his work. Those of you with eagle eyes may have noticed praise for Age of Reptiles in my 2017 book, Recreating an Age of Reptiles, the title of which was chosen as much for its relevance to the comic as it's palaeontological and paleoartistic connotations. Though I don't think Age of Reptiles can qualify as pure palaeoart on grounds of taking a few too many artistic liberties with palaeontological data, it contains many lessons about effective depiction of fossil animals and, 25 years on, I still regard it as some of the best 'palaeontologically-inspired art' (as opposed to entirely science-led palaeoart) out there.

Revisiting some of my 20 year old drawings (I would have been about 13 when I drew these) shows many Delgadoisms. These would all have been influenced by Tribal Warfare, I didn't have The Hunt. The tree outlines, cliffs, waterfalls with sharply defined mist, the hatched scalation, eye shapes and so on were my best attempts to execute an Age of Reptiles style.

In just under a month I have a new book out: The Palaeoartist's Handbook: Recreating Prehistoric Animals in Art, published by Crowood Press. This is a big (280 x 220 mm, 224 pages), full-colour, densely illustrated soft back entirely dedicated to the subject of palaeoartistry: its history, methods, execution and philosophy. It's going to be available internationally from the 27th of August in both physical and digital formats, and online retailers are already taking pre-orders for the cover cost of £22 or less (Amazon sale links: UK/US). I plan on having stock to sell signed copies from my website very soon, and a signing event is planned for TetZooCon 2018 - get your tickets for that here.

With the release impending, I figure it's time to start talking about the book to generate some buzz. The handbook is essentially a palaeoart textbook, containing a history of the genre, an overview of the process of reconstructing an extinct animal, notes on the life appearances of popular extinct taxa, discussions about the artistic and scientific requirements of the discipline and giving practical advice to aspiring palaeoartists. The goal of the book is to be accessible to newcomers while also interesting to veterans and enthusiasts. Sections demystifying geological and palaeontological jargon or introducing important concepts (finer divisions of geological time, phylogenetic bracketing etc.) should be useful to those just entering the discipline, while the detailed discussions, diagrams and citations should interest enthusiasts and professionals.

Emily Willboughby's Microraptor welcomes you to the first chapter of the handbook. I'm very happy with the overall look of the book: it has a good text/figure ratio, is suitably 'dense' without being cluttered, and has lots of nice details like the colour graded panels beneath the chapter openers. The designers have done a really good job.

The idea for the handbook came in January 2016 when I was reading Jackie Garner's excellent Wildlife Artist's Handbook(2013, Crowood Press). It occurred to me that, like conventional natural history art, palaeoart has a long history, its own theory and methods, good and bad practise, as well as a large body of practitioners, and yet we lack texts which discuss palaeoart as a learnable skill or discipline. Virtually all palaeoart books are collections of artwork, historic overviews or 'how to draw dinosaur' volumes, the latter often being of dubious scientific merit. The most detailed discussions of palaeoart theory are found in book chapters or articles, but they're limited in detail because of their lack of space. In writing this blog I've found that there's scope for long, detailed discourses on everything palaeoartistic: if even arcane topics such as extra-oral tissues or predicting horn shapes can justify a few thousand words a piece, then writing about the entire discipline would easily fill a book. Being impressed with the quality of The Wildlife Artist's Handbook, I contacted Crowood about creating a palaeoart equivalent and, 2.5 years later, we're almost at that August release date.

As you may expect from a book about artistry, the handbookis heavily illustrated. It has about 200 figures, photographs and paintings, as well as a large number of annotated diagrams. Not all the artworks are my own, however. Though happy to handle the diagrams and many of the paintings myself, I felt it would be inappropriate to illustrate the book exclusively with my own work - I fear giving the impression of putting my own work on a 'here's how to make palaeoart' pedestal. To that end, I reached out to eight of the most talented and interesting palaeoartists working today: Raven Amos, Julius Csotonyi, John Conway, Johan Egerkrans, Scott Hartman, Rebecca Groom, Bob Nicholls and Emily Willoughby, each of whom graciously donated several pieces of artwork. Their contribution not only makes the book a heck of a lot prettier but also demonstrates a broad stylistic range. The list of contributing artists could easily have been twice as long but, as I'm sure you can appreciate, finding content for this book was never a problem: fitting it all into a reasonably sized package was. Indeed, I had to request more words from the publishers midway through writing and the project ended up being 20,000 words longer than originally intended. This is not to say that the book is cluttered or over-stuffed - to the contrary, I actually find the layout quite comfortable to look at - but simply that we really pushed this one as far as we could go.

Contents page for The Palaeoartist's Handbook. Much of the book is devoted to the reconstruction process, but many other topics - history, composition, professional practise etc. - also feature.

Questions about the handbook's content are best answered with a tour through its chapters. The book opens with a chapter introducing the genre: its scope and depth, its bias towards charismatic fossil vertebrates and how we might distinguish palaeoart from other visual media pertaining to extinct animals. Much focus is given to the line between palaeoart and palaeontologically-inspired art. This subtle distinction is an important one, being the cause of much frustration and confusion among those of us who care about realistic depictions of the past and public education. Ultimately, we have to concede that the creative forces behind the prehistoric animals of movies and toys are rarely on the same page as us: they aren't making 'palaeoart', but 'palaeontologically-inspired art'. These are works that use preferred and marketable aspects of palaeontology to achieve a goal, but ignore components that conflict with their objective. A take home from this is that anyone seriously wanting to be considered a 'palaeoartist' needs to create art of extinct subjects based on evidence and data, not gut feelings, what the latest Jurassic movie is doing, or what we think looks cool.

Chapter 2 is one of my favourite parts of the book: a history of palaeoart from the pre-scientific period right up to the modern day. So many histories of palaeoart are short and selective, often jumping from Duria Antiquior to Hawkins' Crystal Palace models, saying hello to Knight and Burian and then calling it a day. Such treatments omit many important details in the development of palaeoartistry - and I'm not just thinking about the reinvention of palaeoart inspired by the Dinosaur Renaissance. It should be more widely appreciated, for instance, that De la Beche's Duria Antiquior is not the oldest piece of palaeoart. It is widely labelled with this title but a number of works undeniably qualifying as palaeoart pre-date it by 30 years. De la Beche's painting broke new ground in some respects, but the terrain had already been cracked by several other scholars and artists. Another example: historic overviews often focus so much on Knight that they overlook other significant developments taking place in the early 20th century, such as the invention of hybrid 'scientist-palaeoartists' and their strong influence in the genre. While Knight was painting murals Harry Seeley was publishing Dragons of the Air (1901) and Gerhard Heilmann was producing The Origin of Birds (1926), books which contained very progressive takes on pterosaurs and dinosaurs and are clear precursors to the way we illustrate these animals today. I've tried to cram the handbook's overview of palaeoart history with as much information as possible and I feel it's a more comprehensive treatment than you'll find in many venues. It also features a brief section on palaeoart prior to science - my recent blog posts on griffins and cyclopes stemmed from research for this section.

Hendry De la Beche's 1830 artwork Duria Antiquior: A more Ancient Dorset: definitely a landmark illustration for palaeoartistry, but not the first piece of palaeoart. The pre-1830 history of palaeoart gets a lot of discussion in the handbook. Image in public domain.

The third chapter is a crash course in how to research palaeoart. This part of the book will hopefully benefit folks who're new to the discipline and struggling to make sense of the often technical information that informs a palaeoartwork, an especially daunting task for those lacking a background in geology or palaeontology. There's a lot of explanatory text in this chapter, explains (for example) what terms like 'functional morphology' and 'stratigraphy' are, giving advice on how to read a cladogram, and outlining why researching geology and fossil provenance are just as important as understanding anatomy. There are also discussions of where to find information relevant to palaeoart and how to verify it reliability. There's a lot of junk and erroneous information out there, especially online, and these tips should help you to sift some useful information from the detritus.

We talk a lot about epidermal correlates at this blog (see here and here for recent examples) but they aren't as widely used as they should be. They're best known in centrosaurine horned dinosaurs thanks to Hieronymus et al. (2009), but occur widely across tetrapods. We're probably getting a lot of reconstructions wrong by ignoring them. Image from Witton (2018).

Chapters 4-8 outline the process of reconstructing extinct vertebrates. Collectively, these chapters represent a major chunk of the book. They start with the prediction of missing anatomies, building skeletal reconstructions and determining plausible postures. Muscles and fatty tissues are then considered, followed by skin: how we can predict skin types when they aren't present in fossils as well as what we can determine from fossil skin itself. A whole chapter is devoted to facial tissues: extra-oral tissues (lips, cheeks etc.), eyes, ears and noses. Our precision for reconstructing animal faces is something of a mixed bag as some features are much easier to predict than others. We have robust means to predict how much eyeball tissue should be visible, the likely positions of reptile nostrils, and when trunks or proboscides were present, but ask about the shape of extinct mammal ears or what sauropod noses really looked like and we're less certain. Chapter eight deals with hot topics like shrink-wrapping and the role of speculation in soft-tissue reconstruction. Both have roles to play in palaeoart, but both can be 'overdone': the handbook has some food for thought about when, and when not, to make use of these conventions.

Chapter nine drills down into the specifics of restoring tetrapod taxa. I originally envisaged this section as being bigger and encompassing more animal types, but non-tetrapods had to be cut to save space. The alternative would have been to include very brief notes on more taxa, but I fear the sin of error through omission: more detail about popular palaeoart subjects seemed the best compromise. Most major tetrapod groups are included, with specific sections on dinosaurs, pterosaurs, marine reptiles, different 'grades' of synapsids, temnospondyls and others.

'Rictal plates' - the structures that cover the corner of tetrapod mouths - are among the topics discussed in the handbook. Though often mentioned in discussion of dinosaur 'cheeks', they also have relevance to suction feeders, such as the placodont Henodus chelyops. Understanding a subject's functional morphology can guide speculative reconstruction of unknown anatomies. Another image from Witton (2018)

The tenth chapter moves away from restoring animals to considering their environment. As with chapter three this section is aimed partly at newcomers, bringing them up to speed on how ancient environments are understood through sedimentology, stratigraphy and palaeoclimatology. This is not to say Chapter 10 is a geology lecture however: it's more a bluffers guide which explains useful terms and phrases to allow non-geologists to glean information from research papers on the palaeoenvironment of their subject species. Plants are also briefly covered in this chapter. I'm afraid the handbookis not the text that overturns palaeoartisty's general short shrift to palaeobotany, but there is guidance for how to research ancient floras as well as some need-to-know information about plant evolution.

Raven Amos'Nemegt Sunrise shows an entirely typical palaeoart topic - a foraging dinosaur (specifically, Conchoraptor) - but in awesome style. Palaeoart which is scientifically credible but strongly stylised is relatively new to the discipline. Will it become more widespread in future? Raven's excellent image features prominently in the book.

A page from Chapter 9's mosasaur section. Diagrams or illustrations such as these appear on almost every page of the book. When I started the book I figured I'd mostly use 'off the shelf' art, but I ended up creating a lot of new images to illustrate points made in the text. This is why is took two years to write, folks.

The Palaeoartist's Handbook: Recreating Prehistoric Animals in Art, will be available internationally on August 27th, published by Crowood Press. Pre-orders can now be made at Amazon (UK/US) and at other retailers.

Enjoy monthly insights into palaeoart and fossil animal biology? Consider supporting this blog with a monthly micropayment, see bonus content, and get free stuff!

My work - including the writing of educational books like the handbook - is supported through Patreon, the site where you can help online content creators make a living. If you enjoy my content, please consider checking out my Patreon site - subscriptions start at $1 a month. That might seem pretty trivial, but if every reader pitched that amount I could work on books, artwork and other educational content full time. In return, you'll get access to my exclusive Patreon content: regular updates on research papers, books and paintings, including previews of another upcoming book. Plus, you get free stuff - prints, high quality images for printing, books, competitions - as my way of thanking you for your support. As always, huge thanks to everyone who already sponsors my work - without your help, the Palaeoartist's Handbook may not exist.

Miocene pelagornithid Pelagornis chilensis parents and chicks, what fantastic animals they must have been. Note the lack of pseudoteeth on the chicks, recent work suggests they didn't develop until the cusp of adulthood. Not that I drew them in and removed them at the last minute when researching this post, of course.

Given how often I've written about giant pterosaurs, it's peculiar that I've never thought to cover the only flying animals which have ever come close to challenging their size, the pelagornithids: long-winged, often gigantic birds which attained wingspans exceeding 6 m. And yes, a 6 m wingspan is a metric that many pterosaurs - not even just the big azhdarchids - would find endearingly cute, but it's the largest wingspread of any bird and well above the size of any living flying animal. In public engagement pelagornithids are mostly wheeled out to gawk at their size and weird pseudoteeth before being put away again, but there's lots of fascinating anatomy under those big wings, and they deserve a longer period in the spotlight.

Pelagornithids - which are sometimes called (the now defunct name) "pseudodontorns" - were a long-lived and globally distributed group, their fossils ranging from Palaeocene - Pliocene rocks of Eurasia, both Americas, Africa, Antarctica and New Zealand (Mayr and Rubilar-Rogers 2010; Bourdon and Cappetta 2012). They were a group of large-bodied pelagic soarers, seemingly adapted for extended periods of flight over seas and oceans. Most of their fossils are - as is typical for birds - pretty fragmentary, but a number of species are are relatively well represented, especially members of the genus Pelagornis. Their soft-tissue anatomy is virtually unknown, save for primary wing feather impressions associated with the holotype of P. orri (Howard 1957).

Many pelagornithids are known from single bones or a few pieces of skeletal shrapnel. In having good cranial and limb material, the Oligocene taxon Pelagornis sandersi is among the better known species. Note the difference in size of the humerus (e-f) vs. the hindlimb bones (j-q; femur is j-k, tibiotarsus is l-m, and carpometatatsus is o-p). From Ksepka (2014).

Much uncertainty and confusion surrounds the composition of pelagornithid taxa with numerous genera being considered invalid or synonymous with others. This problem is rooted in over-enthusiastic naming of undiagnostic, often crushed scraps of bone as well as a lack of comparable anatomies between holotypes. Evaluation of Neogene pelagornithid material has suggested that most genera are weakly supported, leading Mayr and Rubilar-Rogers (2010) to suggest that all Miocene and Pliocene taxa (which includes Osteodontornis, Pseudodontornis, Neodontornis and possibly Cyphornis) should be sunk into Pelagornis. It's a little difficult to say how speciose Pelagornithidae is given the fluidity of their taxonomy but, as a rough figure, it seems to be composed of a little over a half-dozen Neogene Pelagornis species and a handful of Palaeogene representatives. All pelagornithids share the same basic "long-winged and pseudotoothed" bauplan, but the characteristic anatomies and proportions of the group are most expressed in later taxa, such as Pelagornis.

The confusion over pelagornithid systematics is not confined to generic relationships. Their placement among other birds has been the source of much discussion and controversy, and it's perhaps best to regard their affinities as currently uncertain. Initially regarded as possible relatives of Pelecaniformes (classically thought to contain pelicans, cormorants, gannets and so on - the situation has changed since then), Procellariiformes (tube-nosed birds, including albatrosses) or Ciconiiformes (storks and allies), Bourdon (2005) found stronger evidence linking pelagornithids with Anseriformes - the same group that includes ducks, geese and screamers. Numerous features of the skull and forelimb support this affinity, as do some features of skull development (Louchart et al. 2013). An affinity with waterfowl might seem bizarre for these ocean-going giants but Anseriformes have a long and varied evolutionary history: this is the same branch of avian evolution that (probably) begat the giant, flightless gastornithids and mihirungs, as well as the wader-like Presbyornis. Viewed from a geological perspective instead of a modern one, Anseriformes are not just birds that honk and quack.

But while an anseriform affinity for pelagornithids has not being dismissed out of hand, the idea is not without critics. Some pelagornithid anatomies - such as their sterna - are not anseriform like (Mayr et al. 2008), and other features imply a position outside the anseriform-galliform clade (Galloanserae: crudely, the duck-chicken group) without qualifying for entry into Neoaves (all living birds but ratites and galloanserans) (Mayr and Rubilar-Rogers 2010; Mayr 2011). So while these studies broadly agree that pelagornithids emerged from fairly rootward stock among Aves, and that they are not closely related to any modern soaring birds, further work, and maybe more fossils, are needed to clarify their actual phylogenetic position.

Pelagornithid primary wing feather impressions associated with the holotype of Pelagornis orri. It's not known is these represent the longest feathers of the wing, but they still have a useful role to play in reconstructing pelagornithid wingspans. From Howard (1957).

Size-off: Pelagornithids vs. Argentatvis

All pelagornithids are characterised by large size with even the earliest, smallest taxa being comparable to big albatrosses in wingspan (Bourdon 2005). But how big did they get? I know several readers are already sharpening their comment knives about my introduction suggesting that pelagornithids are avian wingspan record holders, thinking I've forgotten about the giant, 7 m wingspan Miocene teratorn Argentavis magnificens. But that's not a mistake: pelagornithids really should be considered the record holders for avian wingspans, and Argentavis isn't as big as most people imagine.

Classic image of teratorn researcher Kenneth E. Campbell posing with a 25ft wingspan (7.62 m) silhouette model of Argentavis magnificens at the National History Museum of Los Angeles. Alas, Argentavis wasn't quite as big as depicted here. From Campbell (1980).

Some giant pelagornithids, such as Pelagornis chilensis, are unusual among giant fossil fliers in being represented by relatively good skeletal material and their feathered wingspan estimates of 6 m or more can be considered trustworthy, reliable figures. Giant Argentavis magnificens, on the other hand, are known from fragmentary remains and some degree of uncertainty surrounds their wingspans: estimates have ranged from 5.7 to 8.3 m (e.g. Campbell and Tonni 1983; Chatterjee et al. 2007). Recent workers have suggested that the lower range of these estimates is more likely. When describing P. chilensis, Mayr and Rubilar-Rogers (2010) noted that the 82 cm long humerus of their pelagornithid was vastly bigger than the 57 cm long Argentavis humerus, and that scaling the latter to proportions seen in smaller teratorns yields a wing skeleton length of 183 cm. If so, the bony wing spread of the largest Argentavis might have struggled to reach 4 m, and this is before assuming any flex in the wing bone joints. And no, the addition of feathers does not bring Argentavis into record-breaking territory. Ksepka (2014) predicted that the primary feathers of Argentavis would need to be about 1.5 m long to reach a 7-8 m wingspan, a length that would exceed the primary feather: wingspan ratio of all living birds as well as contradict the observation that primaries tend to scale with negative allometry against wingspan. Accordingly, Ksepka (2014) suggested Argentavis was more reliably sized at a 5.09 - 6.07 m wingspan, with estimates at the lower end of that range being predicted in most models. In contrast, the wing skeletons alone of P. chilensis and P. sandersi easily exceed wingspans of 4-5 m, and the addition of conservatively estimated feather lengths easily raise these wingspans into the 6-7 m range.

Skeletal reconstructions of giant pelagornithids: the holotype of Pelagornis chilensis (ventral view) and P. sandersi (dorsal view). That bird to the right of the image is a little thing called the wandering albatross, which has the largest wingspan of any extant flying bird. Pelagornithids must have been amazing to see in life. Images from Mayr and Rubilar-Rogers (2010) and Ksepka (2014).

Despite their size, pelagornithids were not heavy animals. Mass estimates for giant pelagornithids are in the region of 16–29 kg for P. chilensis and 21.9–40.1 kg for P. sandersi (Mayr and Rubilar-Rogers 2010; Ksepka 2014). Given that these birds are twice the size of albatrosses, the fact that their predicted masses match, or only double, the maximum masses of extant flying birds are surprising. Remember that mass increases by a factor of eight for every doubling of a linear dimension so, if we scaled wandering albatross (using masses given at Wikipedia) to Pelagornis proportions we’d expect a mass of 50-96 kg - well above those predicted figures. The pelagornithid weight-watching secret seems to lie in their unique wing proportions: even more than albatross, pelagornithids have extremely long wings compared to the rest of their bodies, and can thus attain giant wingspans while keeping their masses low. Similar tactics were also exploited by giant pterosaurs: maximising wing area while keeping the body small is a great way to maintain volancy at large size. Extremely thin walled bones also helped pelagornithids maintain low masses (and also explains why so many pelagornithid fossils look like they've been hit with a bulldozer).

Biological sailplanes

The largest pelagornithids were of a size which exceeds some theoretical flight limits for albatross-like birds (e.g. Sato et al. 2009), though the plainly obvious flight adaptations of their skeletons suggest this problem lies with our calculations and not the concept of pelagornithid flight itself. Indeed, glide analyses of P. sandersi indicate a supreme soaring capability with a very low sink rate (the rate at which altitude is lost during gliding) and high glide speeds, a combination that would facilitate extremely wide-ranging, energy efficient flight (Ksepka 2014). Their flight performance seems generally more akin to that of albatross than other pelagic birds, so reconstructions of pelagornithids riding air currents between waves, buzzing along the water surface and cruising on ocean winds seems sound. Reduced hindlimb proportions indicate that pelagornithids were probably not capable walkers or runners however, and we might envisage them only landing infrequently, perhaps most commonly when nesting. Curiously flattened and wide toe bones recall those of birds which use their feet as air brakes when landing (Mayr and Rubilar-Rogers 2010; Mayr et al. 2013), and may also have aided stabilisation on land (Mayr et al. 2013).

Predictions of glide ability and lift:drag ratios in P. sandersi from Ksepka (2014). Note how both models compare very well to albatross flight (black), but less well with frigate bird (red) or raptor flight (green).

Maintaining flight is a relatively easy part of aerial locomotion: how pelagornithids became airborne is trickier to fathom. This is mostly because of several indications of a limited flapping ability in the largest Neogene species, which are also the ones that would struggle the most with launch. Scaling of muscle energy availability predisposes all large flying animals to a relatively limited flapping capability and, like all fliers operating at the upper limit of their respective bauplan, pelagornithids likely relied on short-lived bouts of powerful anaerobic muscle activity to perform flapping (Ksepka 2014). But there is some question over whether they could flap their wings at all: several osteological features suggest pelagornithids had reduced shoulder/humeral motion (including a lack of rotatory capability) and lessened downstroke musculature (Mayr et al. 2008). A lack of dynamism is also seen elsewhere on the wing in that the articulation for the alulua was weakly developed, prohibiting spread of this structure during takeoff and landing (Mayr and Rubilar-Rogers 2010). The alula, when extended, allows the wing to function at higher angles of attack (the angle of the wing relative to the direction of airflow) and is thus very useful in initiating flight, controlling landing and general aerial manoeuvrability. Its immobility in pelagornithids would have impacted their range of flight dynamics quite considerably.

The large size of pelagornithids means that a very limited, maybe absent flapping ability may not be as detrimental as we intuitively predict. Flapping motions - both frequency and amplitude - reduce against increasing wing area and flight speed (the latter being predicted as high for any giant flier) so, as the largest flying birds of all time, pelagornithids may not have missed flapping as much as you'd think. But nonetheless, a significantly reduced flapping capacity and limited alula motion may have demanded fairly specialised launch and landing behaviour. Pelagornithids may have been limited to launching by simply extending their wings and using running, gravity or headwinds to find sufficient glide velocity. Landing, by contrast, may have involved low-angle approaches, slowing as much as possible (a dangerous game, as slower gliding also brings higher sink rates) and ditching to the ground. I can entirely believe that undignified semi-crash landings were common in this group.

If our understanding of pelagornithid flight is accurate, typical seabird behaviours like cliff-nesting - demonstrated here by northern gannets (Morus bassanus) - can be ruled out. Long winged seabirds are not the most agile fliers, but many still have enough control over their initiation and cessation of flight to land on small ledges. Pelagornithids trying this may have ended up splattered, Wile E. Coyote-style, on the side of a cliff. Photo by Georgia Witton-Maclean.

Being so light relative to wingspan would assist in both takeoff and landing, but nevertheless question marks hang over their ability to achieve flight in some conditions, such as escaping water (Ksepka 2014). Perhaps, like frigate birds, pelagornithids avoided entering water (though the former struggle with water escape because of waterlogged feathers rather than restricted flapping kinematics). I wonder if this is the case however, it being historically proposed that (unrealistically lightweight) giant pterosaurs could achieve flight from water by simply spreading their wings and catching wind (e.g. Bramwell and Whitfield 1974). The predicted pterosaur masses, wingspans and wing area models used in these old pterosaur studies are not far off those modelled today for giant Pelagornis: if so, could pelagornithids have escaped water with the same wing-spreading trick? It would be interesting to see this modelled biomechanically for a pelagornithid-specific model. On land, we may assume that pelagornithids favoured open space that permitted full 6-7 m wing spreads for launching and landing, and it would not be surprising if they favoured windy, elevated coastal regions that provided environmental launch assistance. I'm not sure what their prospects for flight in continental habits are, but it probably wasn't good: they almost certainly stuck to oceanic soaring, as suggested by the skew of their fossils to marine sediments.

They're only pseudoteeth, but I like them

We’ve made it all the way through this post without discussing the other characteristic anatomy of pelagornithids: their ‘pseudoteeth’. These structures are bony outgrowths of the jaw bones which strongly resemble actual dentition, though histological studies have verified that they lack all tissues associated with true teeth (Howard 1957; Louchart et al. 2013). It’s thought that pseudoteeth compensated for a well-developed hinge in the lower jaw that permitted wide horizontal bowing during feeding. This allowed for large prey to be swallowed, but compromised overall jaw integrity and potentially risked the loss of slippery seafood prey. A set of variably sized spikes along the margins of the beak is a great way to ensure snagged foodstuffs - thought to be mainly surface-seized fish or squid - did not slip from their beaks. Though many of the spikes are hollow, jaw bone surficial textures indicate that the entire jaw - pseudoteeth and all - was covered with a cornified sheath typical of other bird beaks (Louchart et al. 2013). As we discussed when looking at mammal horns, that’s a pretty potent combination for maximising lightness with strength, though the bone forming the pseudoteeth was mechanically weak and, despite their ferocious appearance, they were not adapted for tackling large, formidable prey (Louchart et al. 2013).

Holotype skull of P. chilensis in lateral view: check out those pseudoteeth. From Mayr and Rubilar-Rogers (2010).

That pelagornithid teeth functioned well as fish-grabs is suggested in their similarity in size and distribution to the dentition of other fish eaters, including certain crocodylians, large predatory fish, pterosaurs and temnospondyls. Quite how pelagornithids caught their prey is not well understood: if they could enter the water, they may have foraged from the water surface or dived; if not, they may have snatched prey from the water surface or stole it from other birds. Further research into pelagornithid flight capabilities and launch kinematics would narrow down this range of possibilities.

Recent studies have shown that pseudoteeth erupted from the jaw relatively late in pelagornithid growth (Louchart et al. 2013), meaning juvenile Pelagornis would have looked like regular, cute baby birds before developing their toothy smiles as adults. This has several interesting implications for pelagornithid growth and ecology. The first is that the cornified beak tissue covering their jaws must not have hardened until after the teeth had fully developed (recall from a previous post that cornified sheaths, on account of being inert, dead tissue, can’t be easily modified once deposited). This characteristic is not common among birds, but occurs in a number of Anseriformes. This observation is not a deal clincher for the pelagornithid-anseriform phylogenetic hypothesis, but it's an interesting connection nonetheless. Secondly, studies show that the emerging pseudoteeth were relatively delicate and potentially unable to withstand stresses imparted by thrashing fish or squid until late in development. This being the case, Louchart et al. (2013) proposed that pelagornithids might have been altricial, feeding regurgitated food to their offspring until they were fully grown and able to forage for themselves; or else that the juveniles were foraging on different foodstuffs. Altriciality would be unusual behaviour for a stem-neoavian as most bird species of this grade have precocial offspring that feed themselves straight after hatching. Insight into these hypotheses would be provided by fossils of juvenile pelagornithids but these remain extremely rare. I wonder if these animals were like living pelagic birds and nested atop cliffs in isolated offshore settings? If so, I wouldn’t hold your breath waiting for fossils of their hatchlings.

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References

• Bourdon, E. (2005). Osteological evidence for sister group relationship between pseudo-toothed birds (Aves: Odontopterygiformes) and waterfowls (Anseriformes). Naturwissenschaften, 92(12), 586-591.
• Bourdon, E., & Cappetta, H. (2012). Pseudo-toothed birds (Aves, Odontopterygiformes) from the Eocene phosphate deposits of Togo, Africa. Journal of Vertebrate Paleontology, 32(4), 965-970.
• Bramwell, C. D., & Whitfield, G. R. (1974). Biomechanics of Pteranodon. Phil. Trans. R. Soc. Lond. B, 267(890), 503-581.
• Campbell Jr, K. C. (1980). The world's largest flying bird. Terra, 19(2), 20-23.
• Campbell Jr, K. E., & Tonni, E. P. (1983). Size and locomotion in teratorns (Aves: Teratornithidae). The Auk, 390-403.
• Chatterjee, S., Templin, R. J., & Campbell, K. E. (2007). The aerodynamics of Argentavis, the world's largest flying bird from the Miocene of Argentina. Proceedings of the National Academy of Sciences, 104(30), 12398-12403.
• Howard, H. (1957). A gigantic" toothed" marine bird from the Miocene of California. Santa Barbara Museum of Natural History, Department of Geology Bulletin, (1), 1-23.
• Ksepka, D. T. (2014). Flight performance of the largest volant bird. Proceedings of the National Academy of Sciences, 111(29), 10624-10629.
• Louchart, A., Sire, J. Y., Mourer-Chauviré, C., Geraads, D., Viriot, L., & de Buffrénil, V. (2013). Structure and growth pattern of pseudoteeth in Pelagornis mauretanicus (Aves, Odontopterygiformes, Pelagornithidae). PloS one, 8(11), e80372. 
• Mayr, G. (2011). Cenozoic mystery birds–on the phylogenetic affinities of bony‐toothed birds (Pelagornithidae). Zoologica Scripta, 40(5), 448-467.
• Mayr, G., Hazevoet, C. J., Dantas, P., & Cachão, M. (2008). A sternum of a very large bony-toothed bird (Pelagornithidae) from the Miocene of Portugal. Journal of vertebrate Paleontology, 28(3), 762-769.
• Mayr, G., & Rubilar-Rogers, D. (2010). Osteology of a new giant bony-toothed bird from the Miocene of Chile, with a revision of the taxonomy of Neogene Pelagornithidae. Journal of Vertebrate Paleontology, 30(5), 1313-1330.
• Mayr, G., Goedert, J. L., & McLeod, S. A. (2013). Partial skeleton of a bony-toothed bird from the late Oligocene/early Miocene of Oregon (USA) and the systematics of neogene Pelagornithidae. Journal of Paleontology, 87(5), 922-929.
• Sato, K., Sakamoto, K. Q., Watanuki, Y., Takahashi, A., Katsumata, N., Bost, C. A., & Weimerskirch, H. (2009). Scaling of soaring seabirds and implications for flight abilities of giant pterosaurs. PLoS One, 4(4), e5400.

If you're the sort of person who's interested in cool stuff like anatomy, evolution and functional morphology, you can't have missed two incredible books published in recent years by author and artist Katrina van Grouw: The Unfeathered Bird(2013) and Unnatural Selection (2018). Although tackling different topics, they are united by their exceptional illustrations of animals in various states of dissection (though mostly skeletonised), lavish design, great production quality and highly detailed yet accessible text. The Unfeathered Bird, which focuses on bird skeletal anatomy and functional morphology, caused ripples in the palaeontological community upon release for being a book which looks at modern birds the way we look at fossil animals. As a monumental book - huge, comprehensive, scholarly and aesthetically spectacular - I'm sure we would all have been happy with more of the same for Katrina's follow up project (The Unfuzzy Mammal, The Unscaled Reptile, The Un... er... skinned Amphibian?) but her latest book, Unnatural Selection, is even more ambitious in scope, detailing how human-controlled animal breeding - the titular 'unnatural selection' - offers a window into the mechanics of biological evolution.

Katrina's new book, Unnatural Selection, available now from Princeton University Press, all the usual online retailers and good book shops.

To celebrate the release of Unnatural Selection a few months ago, I invited Katrina to give an interview about her new book, her art, and her future projects. I do this as a certified van Grouw fan: the Disacknowledgement and I have three van Grouw artworks on our wall in addition to her books on our shelves. Katrina has always skirted the line between author, scientist and artist and, before The Unfeathered Bird, she already had a background in fine art, museum curation (bird skin curation at the Natural History Museum) and book writing. I will fully confess to finding her skills and knowledge intimidating, and was a little frightened of meeting her at the 2015 TetZooCon. After all, if anyone was ever going to expose my work for the hack job it is, it would be this Baroness van Grouw, dual master of detailed anatomy and artistry (lightning flashes, thunder rumbles)*. It turns out that it's actually impossible to be scared of Katrina once you talk to her however, and we remain close friends today.

*Note that Katrina's status as a Baroness is a product of my over-active imagination, not reality. 

There's lots to like about Katrina's books, and I mean no disrespect when saying that they're some of my favourite books simply to look at. Katrina's illustrations - drawn in pencil and then tinted to sepia tones digitally - are truly world class, and her subjects are frequently drawn in lively, life-appropriate poses so that, even when skeletonised or half-dissected, they look very much alive. But it's not only the drawings which make her books exceptional to behold: their size, ivory-coloured pages, font hues and text layouts recall a romanticised age of 19th century museums and scholarship. It's impossible not to think of exhibition halls filled with wooden cabinets, animal bones, taxidermy specimens, curiosities in jars and stuffy formalwear when reading these books. They evoke the atmosphere of a classic age of learning and exploration on every page. It must be stressed how, for their size and quality, both The Unfeathered Bird and Unnatural Selection are exceptionally good value for money ($45-50 cover price, but being sold at £20-25 at Amazon UK, and equivalent in the US). I can only wonder what Mephistophelian deal Princeton University Press has made to to sell these fantastic books at such low costs and, to the poor production editors suffering in the afterlife for making such a deal, know that it was really worth it: I can't think of many books published in recent years that are anywhere near as splendid to look at in this price bracket. For next-level book quality I can only think of tomes like the considerably more expensive (but ultimately disappointing) Paleoart: Visions of the Prehistoric Past (Lescaze 2017), with a cover price twice that of Katrina's books.

Fans of The Unfeathered Bird will be intimately familiar with Katrina's skeletonised birds. Unnatural Selection offers a greater proportion of art devoted to living individuals, including this jungle fowl, the ancestral species of chickens. © Katrina van Grouw.

The strangely deformed skull of a King Charles spaniel. We view this sort of anatomical modification as extreme, and certainly there are welfare issues to consider with many domestic breeds (and not just dogs, as is most widely reported). But many of the extreme (and healthy) breeds we've created are no more bizarre than what we see in nature. In response to questions about pushing animal anatomy too far, Unnatural Selection responds "...look at what nature has done to the sword-billed hummingbird!". However we feel about the ethics of breeding bizarrely proportioned animals, we're not the only evolutionary force behind their creation. From Unnatural Selection, © Katrina van Grouw.

MW. Much of Unnatural Selection discusses the prolonged challenge of having artificial selection viewed as having useful insights on 'natural' evolution. As you argue, this seems peculiar - a bit like a physicist arguing that experiments done on atomic particles in the lab have no bearing on 'naturally occurring' particles. Why do you think this issue has persisted for as long as it has?

KvG. I honestly have no idea. I too used to be what I call a ‘wild animal snob’ i.e. I considered domesticated animals as something man-made and irrelevant, though I’ve come to revise my opinions significantly!

It’s always said that Darwin drew up his analogy between natural and artificial selection specifically in order to introduce to the Victorian public the frightening concept that all the diversity of life could have come about without the need for a divine creator. Like the spoon full of sugar that makes the medicine go down. I suspect, as Darwin later maintained in his revised autobiography, that artificial selection was indeed highly influential in the formulation of his theory.

However, even Darwin had problems accepting that certain radical traits (like achondroplasia, or disproportionate dwarfism, discussed in the next question) could be successful in the evolution of wild animals, so he drew a line between them.

Richard Goldschmidt in the 1940s also made macro mutations appear ridiculous as a result of his ‘hopeful monster’ theory, which may have caused scientists to dismiss selective breeding as useless in comparison with evolution in nature. As you’ll be able to see from the next question, and especially in the book itself, there is actually very strong evidence that the same traits not only occur in wild animals, but may be favoured by natural selection.

Four-winged dinosaurs have evolved a number of times, including at the hands of pigeon breeders (pigeon hindwing, or 'muff' foot, shown at the base of the image). Understanding the genetic changes behind the development of such traits may help us understand the mechanisms through which these evolved in nature, something of obvious interest to palaeontologists interested in development of stem birds and avian flight. From Unnatural Selection, © Katrina van Grouw.

Every so often geneticists make a so-called breakthrough revealing some trend in domesticated animals that sheds light on evolutionary biology, and invariably these ‘discoveries’ have been known already to generations of animal fanciers who breed them. Feathered feet in pigeons are a good example. Every pigeon fancier knows that the wing-like ‘muff’ feet (palaeo-people will know these as ‘hind wings’) are produced by combining two very different foot feather types. Geneticists would do well to pay attention to the knowledge of animal fanciers instead of constantly re-inventing the wheel.

Whereas in past centuries scientists had eclectic interests and would happily turn to the products of horticulture as to botany, or to animal breeding as to wild animals, it seems that the more knowledge that’s available the more blinkered and over-specialised scientists become. I even know molecular geneticists who don’t fully understand Mendelian principles.

Sadly for too many of us, even though the subject of domestication is currently very popular in scientific circles, selective breeding is viewed either with derision or contempt. Fit only for children or eccentric losers, or an emotive subject to stir up the wrath of animal welfare crusaders.

I hope that my book will do something to redress the balance.

Unnatural Selection draws attention to the rapidity of evolutionary modification, specifically how animal form can be dramatically modified in a generation thanks to single genetic changes. You somewhat playfully speculate that this may have had a role in the evolution of some groups - such as the short limbs of mustelids: would you care to suggest - speculatively or otherwise - if any other groups may have benefited from these rapid changes?

That’s right. Though it’s important to stress that the most significant evolutionary change is the result of an accumulation of tiny steps – ‘natura non facit saltus’, as Darwin was fond of saying: ‘nature does not make leaps’. But there are plenty of traits that are single step, all or nothing, changes. Think of the spiralling direction of shells, or horns, for example. Colour aberrations too can result in major phenotypic changes just by the addition or subtraction of a pigment. (Colour ‘aberrations’ are only aberrant when they’re in low numbers in a population, but if they prove to be advantageous, or if they’re isolated in a small gene pool, these aberrations of today can be the colour morphs, races, or even species of tomorrow.)

Vertebral counts are considered a big deal in vertebrate palaeontology, often being used to distinguish different species. It turns out that gaining additional vertebrae is not particularly difficult and is not just the remit of weirdos with extreme numbers of axial elements, like snakes. We've emphasised this trait among species that we harvest for meat, such as landrace pigs, which have several more vertebrae than other breeds. From Unnatural Selection, © Katrina van Grouw.

However, I’m deviating in my preamble. There are lots, and lots, of other examples, besides the short limbs of mustelids, where you can suggest, speculatively, that the evolution of wild animal groups may have been given a ‘leg up’ (no pun intended!) by a single major mutation. The naked neck trait in chickens is a good one - the heat-reducing naked necks of marabou storks, ostriches, and vultures is comparable and is probably caused by the same genetic process. Henny feathering in chickens (where the male has the same plumage as the female with no loss of virility) possibly has a connection with the seasonal eclipse plumage in waterfowl. Mutations that cause winglessness in chickens may sound pretty gross, but moas were totally wingless too, and there’s no fossil evidence to suggest that they ever went through a gradual evolution towards winglessness. Rex fur in rabbits may seem like just a fancy coat type for the show bench, but a similar coat (without directional guards hairs) allows moles to move freely backwards and forwards in their burrows.

The exact same mutations are equally likely to occur in wild as domesticated animals and their success is all down to whether or not they find themselves in an environment that’s favourable. The ‘right’ environment can come in many forms, and it can change, but traits that might not be successful in the wild for whatever reason might just appeal to the whims of fanciers. There are only a finite number of possible traits that will ever be viable, and the same things tend to occur across a wide range of animal groups, albeit expressed differently. Meaning that speculative zoologists would do well to pay more attention to domesticated animals and the traits they exhibit. This is variation in its most likely forms and a very accurate suggestion of the possible directions evolution might take in the future. Again, the initial mutation only needs to be a start. After that, natural selection can refine it further and fine tune it over millions of years to each particular environment.

Both The Unfeathered Bird and Unnatural Selection are simply stunning to look at, but not all your readers will know how instrumental you were to their design, not only creating the text and illustrations, but also having tight control over the page layouts and overall aesthetics. I gather this level of control is not typical in publishing, and it very much makes these your books. Can you tell us a little about your design decisions? 

I’m glad you asked that; indeed, most people assume that the publisher designed the books, which is the more normal arrangement. I’m exceedingly lucky to have a publisher that has so much faith in me.

After nearly two decades of trying to find a publisher for The Unfeathered Bird, I was painfully aware that many people associate anatomy with greyscale diagrams in academic textbooks. I wanted it to be as much a work of art as of science, so changing the colour of the finished pencil drawings to a sepia brown colour, and printing onto ivory-coloured paper was a deliberate attempt to make the book softer and more accessible and to be suggestive of the beautiful historical natural history illustrations of past centuries. This particularly suited the historical theme of both books: The Unfeathered Bird references Linnaeus (this was a clever way to discuss adaptations through convergence rather than adhering to actual phylogenetic relationships) and Unnatural Selection is all about Darwin and Mendel.

The many faces of exhibition homing pigeons, an example of one of the full page spreads from Unnatural Selection. I can't be the only one thinking the Exhibition Homer resembles a certain piece of Therizinosaurus artwork. © Katrina van Grouw.

I had a lot of fun designing the page layouts, especially the chapter opening pages in Unnatural Selection where I’ve shown historical museum specimens against antiquated-looking background paper. And the three coloured images in the book where I discuss pigment changes. The ‘let’s colour a Gouldian finch!’ page was based on Andy Warhol’s Marilyn Monroe prints, with more than a passing nod towards Edward Lear’s coloured birds. This pleased me immensely as John Gould treated Edward Lear very badly in life, so it seemed like justice for Lear!

I'm curious about the poses of many of your subjects. Where you have control over this aspect of their illustration, what helped you decide to pose them a certain way, other than the simple practicalities of "I want to show this feature"?

Many of my decisions were indeed guided by ‘I want to show this feature’. The Unfeathered Bird is all about adaptations, so it was important to show the features that I describe in the text as being particularly adapted to specific behaviour. So it made sense from a scientific and aesthetic point of view to show skeletons actually engaged in that behaviour. Husband put together the majority of the skeletons so it seemed fair to give him an input into the choice of positions. We had endless pleasure discussing the behaviour of birds and arguing (in a friendly way) about which position to choose.

Of course it’s not just the position of the actual skeleton, but the viewpoint I select to draw it from. The diver in the surface swimming position may be fairly straightforward, but I decided on a fish’s-eye view, looking up at it from below, as this was the viewpoint that most clearly showed the boat-like sternum, sideways-angled hind-limbs, and razor-thin legs.

A budgie skeleton passes the mirror test, from The Unfeathered Bird.© Katrina van Grouw.

Some of the poses, although superficially just for fun, are for practical as well as aesthetic reasons. The budgie skeleton looking at itself in a mirror in The Unfeathered Bird was a way of showing both sides of the skull in the same drawing (while making it crystal clear that it’s a budgie!). I did the same thing with the reflection in the water under the porpoising penguin.

I enjoyed playing around with visual references, for example I deliberately showed the avocet in the same position as the RSPB logo; the red grouse was modelled on the whisky label, and the robin on the spade handle was modelled on 90% of the Christmas cards ever printed!

For all the major bird groups I included a page showing the feather tracts, musculature and skeleton of a species all together and for these it made sense to show them like an actual group of birds interacting. So I made up little tableaus: three rooks rushing to get a worm that’s just disappearing underground, or a pigeon’s frantic pouting courtship ritual being completely ignored by the other pigeons only intent on eating.

The skeleton of the mighty auroch, one of my favourite pieces from Unnatural Selection. Katrina didn't pose this individual (it's based on a museum mount) but has really captured its implied energy and mass. We might be looking at a skeleton, but you can almost see the muscles rippling as the animal moves. From Unnatural Selection, © Katrina van Grouw.

I can't look at your anatomical work without wondering how you could translate your skill into palaeoart, and I must not be the only person imagining that a van Grouw restoration of a fossil bird or (based on some of the amazing work in Unnatural Selection) a fossil mammal would be spectacular. Will we ever see a life restoration of a Gastornis, entelodont or Microraptor emerge from your pencils? And can I have a copy if it happens? I'll be your best friend.

Proof, as if it were needed, that Katrina knows her way around skeletons and musculature as well as the best palaeoartists: a parade of cow bottoms. Note the 'double muscle' individual at the end of the row. From Unnatural Selection, © Katrina van Grouw.

Haha, if it happens I promise you you’ll be the first to know! Probably because I’ll need your help to get it right and won’t be brave enough to show it to anyone until you’ve given it your seal of approval! I’m not adverse to the idea, but at the present moment I can’t envisage how it would fit into the planned book projects which, for the foreseeable future, will be purely anatomical.

It’s not entirely impossible though. With the right incentive (for example, producing commissioned illustrations for someone else’s book) I could probably be persuaded.

Katrina's take on Pomarine skuas. I can't be the only person wanting to see this anatomical expertise and style applied to fossil animals. © Katrina van Grouw.

Before your recent books you were best known for your work illustrating fantastic cliffs and seabird colonies. These are amazing images and I understand a number of folks were sad to hear you declare that this part of your artistic life is behind you. Is the door completely closed to this sort of wildlife art, or will it reinvent itself in some form? And if not, what lies ahead? 

When I finished The Unfeathered Bird I was fully intending to return to those sorts of pictures and to being a fine artist again, but I found that I’d moved on. It’s not that I refuse to do the same work; it’s because my heart is somewhere else now. You can’t force it – it has to come from genuine passion. Ever had a persistent ex who tries to convince you that you can fall in love with them again, while you know damned well it ain’t ever gonna happen? It’s a bit like that.

One of Katrina's pre-Unfeathered Bird cliffscapes, St. Abbs Head, Scotland. © Katrina van Grouw.

To be brutally honest I have nothing but contempt people who find it sad that I no longer do the same old work. It shows a lack of respect for my artistic integrity. It wouldn’t be so bad if these were faithful collectors and patrons from my past, but they’re not. Most, in fact, are creative people too so they should understand that creative development is a one-way process and not a matter of personal choice.

Before the rocks/seabird colony drawings I did images of big dramatic birds doing exciting things: fighting and chasing each other and stuff. Then I had a… I guess you’d call it an epiphany, on the cliffs of Hermaness in Shetland, and my work changed to the rocks overnight. And yep, you’ve guessed it, loads of people expressed regret that I wasn’t still doing the same bird pictures…

The important thing to remember is that the illustrations in my books are just that – illustrations. The book’s aren’t art books showcasing my anatomical art; they’re science books – albeit very beautiful ones. Each illustration is just a means to an end and a very small part of the whole. To me the creation of the book – the entire book, from conception to design - is the creative process. What can possibly be finer than bringing into existence an entirely original and very beautiful book?! It ticks all the boxes for me at every creative and intellectual level.

The important thing is to bring all this stuff – pictures or books or whatever - into the world, and the fact that it can’t be relied upon to be repeated forever is what makes it precious. I don’t know what lies ahead, of course, but I think it highly unlikely that I’ll ever return to being a fine artist. But if I do develop in an unexpected direction, promise me you won’t say it’s sad that I no longer produce books!

Katrina's terrifically moody albatross piece, a big dramatic bird doing an exciting thing. A skeletonised version of this image graces the opening of The Unfeathered Bird.© Katrina van Grouw.

In some respects we have arrived at a similar career path from opposing directions: I became an author/illustrator through training as a scientist, while you trained as an artist before authoring science books. You mention in Unnatural Selection that this professional path was not without its struggles, and I can entirely empathise with this. You have to become equally skilled in two, sometimes very different fields and practical issues mean it's not easy to be highly trained in both. Despite these challenges, you've produced two spectacular and very well received books. How have you dealt with the challenge of transitioning from artist to author who does art?

To be fair to myself, although I was deprived of the opportunity of a formal science education, my world before The Unfeathered Bird wasn’t entirely devoid of science, though it was practical rather than theoretical. I was a passionate birder from childhood, and my interests led me to train as a bird ringer in my early 20’s. I took part in long term ringing expeditions to Senegal and Ecuador and was particularly interested in moult and other physiological indicators that ringers use to age and sex birds. Around this time I taught myself taxidermy and began to assemble my own bird skin collection, and also prepared bird skins for museums. The personal project that let to me to make an in-depth study of bird anatomy (beginning with a dead duck I christened Amy) has been told many times already. Ironically, many of these elements are not things that science undergraduates are taught. I know plenty of lettered biologists who have never even touched a scalpel. That was all back in the late 1980’s so the transition into science hasn’t been a sudden recent change.

Anyone can educate themselves. The difficulties lie in the lack of opportunity for discussion with peers and tutors, and the difficulty of accessing academic texts, but it’s certainly not impossible. Harder to throw off is the bias based on your actual qualifications.

A disproportionately dwarfed "ancon" sheep, an example of a single-step evolution. Such sheep are more common than we may expect, and we have to wonder how often such radical changes occur naturally. It's conceivable that major, single-step changes in animal form could be selected for under some natural circumstances, and may have even played a role in 'natural' animal evolution. From Unnatural Selection, © Katrina van Grouw.

What I find most difficult, and personally infuriating, is people’s preconceptions; the tendency to put two and two together to make five. Many people find it difficult to accept that someone with an art background can have a genuinely scientific interest in something. They’ll assume that my anatomical interests have an artistic root – as though I’m inspired by textures and shapes and all that. And of course I do enjoy drawing this sort of thing, but it’s not the reason for it. I try to explain that if I wasn’t producing books, I wouldn’t be drawing skeletons, but no-one wants to hear that. Even on my book tour this year I sometimes had to really fight to be described as an author and not an artist. One venue insisted on advertising my talk: ‘Katrina van Grouw’s evolutionary illustrations’. It was humiliating.

The most damaging element of all this, and one that I find deeply painful, is the assumption that I must rely on Husband to help me with the science in my books, because I’m an artist (‘only’ an artist is the unspoken word here). In fact neither of us have an academic science background, and both of us formerly shared a career as curator of the bird collections at the Natural History Museum. Husband is indeed useful for mounting skeletons, and for his knowledge of domesticated animals, but I’m the one with the interest in evolutionary biology.

All in all it seems to be a lot easier for a scientist to be respected as a self-taught artist than the other way around.

Being self-taught does have its plus side however. Having to learn science the hard way means that I know a lot of the pitfalls and barriers to grasping each subject, so I’m in a good place to explain difficult concepts to others in a clear way. It’s made me a really good science communicator.

Given that this is primarily a palaeontology-led blog, I feel we should end with the news that The Unfeathered Bird will be making a return at some point in future, and will have a significant palaeo-themed component this time. Can you give any hints as to what to expect?

Yep, that’s right. I actually signed the contract to do a second edition of The Unfeathered Bird last spring, but I haven’t quite gotten into gear with it yet due to all the work of production and publicity for Unnatural Selection.

It’ll have an extra 96 pages, making a total of 400. You’ll remember that the first edition is divided into two sections: ‘generic’ – talking about birds in general - and ‘specific’ – talking about particular bird groups. The new material will mostly go into the generic section, looking at the definition of what makes a bird, what birds are and aren’t, and of course a whole load of stuff about bird evolution.

Red throated diver skeleton, from The Unfeathered Bird. A new edition will be with us one day! © Katrina van Grouw.

Bird wings will be compared with bats and pterosaurs (and Yi qi?). There’ll be stuff about feather evolution (maybe I can get a palaeo reconstruction in here somewhere...), lots of stuff about the loss of digits, the rotation of the wrist, shortening of tails, lengthening of necks, and the orientation of thighs. And you can expect a few active maniraptoran skeletons doing maniraptoran things.

In addition to this I’ll be replacing some of the drawings in the ‘specific section’; adding some more and giving the existing ones a good polish. I’ll be completely re-writing all the text and replacing the short family sections with entire chapters. The total word count is estimated to be around 110,000 words, in comparison with the 46,000 of the first edition. The science will be better, though still easily accessible. There’ll probably be a new jacket design, but almost certainly still showing peacocks. And I’m guessing the original title will be followed by a subtitle (suggestions on a postcard, please).

The Unfeathered Bird NEEDS a palaeo-themed component. It’s incomplete without it, and any reviewer who wanted blood could easily and justifiably have torn the first edition into shreds. But that didn’t happen. Instead the palaeo-world received us with open arms and heaped praise upon us. It was possibly the most humbling experience of my life. This new edition is my way of expressing my gratitude.