Trunk or no trunk, small or giant ears, long or short neck... what did the giant rhinocerotoid Paraceratherium really look like?

Giant, Oligocene rhinocerotoids Paraceratherium transouralicum engage in some early morning flirting. Because, in rhino speak, playing hard to get involves shoulder barges and head-butts.

Depictions of the giant indricotherines, relatives of modern rhinoceros that lived across mid- and eastern Asia during the Oligocene, have varied over time. We've known about these animals - which are part of a longer-lived (Eocene-Miocene) indricotherine lineage that includes a number of smaller, almost okapi-or horse-like species - for over 100 years and they have become regular fixtures in museums, books and those rare documentaries which offer glimpses into ancient life outside of the Mesozoic. Any yet, when we think of our favourite indricothere paintings - including those by our most celebrated mammalian palaeoartists such as Knight, Burian, Anton, and Buell - they often differ markedly in their depiction of these 15-20 tonne animals. Most notably, their neck proportions, overall robustness, the development of a proboscis or trunk, and - most recently - the size of the ears are all inconsistent. Why are these animals so differently depicted, and should we rule out some of the anatomies we've seen in palaeoart in the last century? Having faced these questions recently when asked to restore this animal myself (above), I thought I'd share some of what I learned in my research here.

The obligatory note on nomenclature

It almost seems tradition that any article or paper on indricotherines requires an aside on their confused taxonomy. As has been the case for decades now, the taxonomy and systematic nomenclature of these giant rhinocerotoids are a matter of ongoing discussion. It is widely appreciated that several giant indricotherine species from roughly contemporaneous Oligocene Asian sediments can be identified, but how many species they represent, and how they are related to each other, is not clear. At least seven generic titles and many more species names have been given to the largest of these animals over the years (Indricotherium and Baluchitherium are perhaps the most famous generic labels), but some authors (e.g. Lucas and Sobus 1989, Prothero 2013) tidy all or most of these taxa into three species of the oldest established genus, Paraceratherium. Arguments persist, however, that at least one other, perhaps slightly smaller genus existed, Dzungariotherium (Qiu and Wang 2007). Geologically older indricotherine genera such as the Eocene Urtinotherium are also wrapped into these discussions as remains attributed to the Oligocene genera are sometimes argued as having greater affinity to these older taxa (Prothero 2013).

This confusion is sometimes framed as a 'lumper/splitter' philosophical distinction, but it does not help that the fossil record of these giant rhinocerotoids is far from exemplar: giant indricotherine specimens can be fragmentary, of starkly contrasting size with one another, and many suffer from distortion. The fact that 20th century indricotherine science developed with Asian and American teams working largely in isolation, with limited access to certain specimens and literature, has also contributed to the confused history of this group. Those interested in the history of indricothere taxonomy should check out Prothero (2013) for an overview. For now, it will serve us to simply state that the best known, biggest and most famous of these animals currently resides as the taxonomic address of Paraceratherium transouralicum. This is the species most of us think of as 'the' giant indriotherine as well as the taxon that has carried both the Indricotherium and Baluchitherium label at one time or another. It's also the focus of most artwork of giant rhinoceratoids, and thus forms our primary interest here.

Giant rhino, bulky giraffe or giant workhorse?

One reason we see such variation in indricotherine appearance is that researchers have produced vastly different interpretations of its anatomy in the last 100 years. But unlike, say, dinosaurs, where older reconstructions have been (for the most part) abandoned in favour of newer, more accurate interpretations, educators and researchers continue to publish skeletal reconstructions published in the 20s and 30s despite our improved knowledge of indricotherine anatomy, documented criticisms of these older works, and the availability of more modern, theoretically better-informed reconstructions.

Many readers may be aware that the first reconstruction of Paraceratherium, published by Osborn (1923a),showed a form not too far off a giant rhinoceros - a heavyset, short-necked animal with a deep torso and short legs. Osborn published a revised version almost immediately after his first effort, which had a much longer neck and longer legs thanks to data provided by additional fossil material (Osborn 1923b). A shorter-necked version was then produced by Granger and Gregory (1935, 1936), who scaled the remains of numerous, differently-sized individuals from a range of collections to create their robust, gigantic take on indricotherine anatomy. Although this reconstruction has been quite influential, Fortelius and Kappelman (1993) have been critical of the scaling methods used by Granger and Gregory, calling their interpretation 'a highly speculative creation indeed'.

Paraceratherium has been variably reconstructed over the years, with particular disagreement over how long the neck was compared to the body. So far as I can tell, a consensus on the life appearance of these animals has yet to be reached.

A third contrasting reconstruction was published a few decades later by Gromova (1959), based on a composite mounted skeleton in the Paleontological Institute, Russian Academy of Sciences. This reconstruction, executed by N. Yanshinova, was accompanied by several wonderful muscle and skin reconstructions which palaeoart fans will not want to miss. Both the mount and reconstruction show a gracile, giraffe-like form with a remarkably long neck and, in being based on a relatively complete set of giant indricotherine remains, some have argued it is a superior take on indricotherine anatomy than those produced by Osborn, or Granger and Gregory (Fortelius and Kappelman 1993). The most striking aspect of this reconstruction is its very long neck. We have to stress that this is extrapolated from a few incomplete cervicals associated with postcranial material, and its exact length remains uncertain - a complete set of neck bones remains elusive for Paraceratherium. This is another reconstruction which has been quite influential (helped, no doubt, by its apparent basis for the BBC's Walking with Beasts 'Indricotherium') but, again, it has not escaped criticism. Paul (1997) suggested that multiple aspects of this mount and reconstruction were erroneous, including the length of the neck, the size of the pelvis and depth of the ribcage, the length of the feet, and the ratio of the humerus and femur, as well as the fully erect posture of the limbs.

And so we turn to another indricotherine skeletal reconstruction, produced by Paul (1997). This restoration incorporated data from the same specimens used in the efforts above and came out somewhat 'averaged' between the more heavyset restorations of the early 20th century and the gracile interpretation of the 1950s. It looks, in overall form, more like a giant workhorse than it does a giant rhino or bulky giraffe. Paul (1997) provides some discussion of the reconstruction process - this is worth a read if you're interested in the life appearance of Paraceratherium and its relatives. Paul's interpretation has, to my knowledge, escaped criticism to date and, to the contrary, Larramendi (2016) described this reconstruction as 'accurate', although did not elaborate on why it should be considered superior to older efforts.

The million dollar question here is obvious: which one of these different takes on Paraceratherium is 'right'? To be honest, I'm not sure. The situation is compounded by the fact that a lot of indricotherine literature is obscure, that the specimens fragmentary and that many of them await description. I was hoping that Donald Prothero's recent (2013) book Rhinoceros Giants, which is solely dedicated to Paraceratherium, would provide some insight on this matter, but it's not a great help here - it provides no real evaluation on the different reconstructions and does not even mention Paul's 1997 effort. My work above is primarily based on Paul's (1997) skeletal but this is largely because of principle rather than real insight. Paul's work is the most modern and, of course, he's made a career out of reliably reconstructing extinct animals. The brief endorsement from Larramendi (2016) helps here too, of course, but a longer discussion of the relative merits and detriments of each interpretation would be useful. Opinions from others with more insight into this matter are welcome in the comments below.

A tapir-like proboscis... on a rhino?

Turning our attention to the face, did Paraceratherium and its relatives have relatively short-lipped faces like those of rhinos, or long, mobile proboscides like their more distant relatives, the tapirs? Despite mammal lips and nasal tissues being highly fleshly and thus only rarely entering the fossil record, this is a surprisingly easy question to answer. Whether rhino, tapir or anything else, a suite of osteological characters seem to correlate well with the presence of proboscides. Briefly summarised, these are: narrow snouts; retraction of the nasal openings towards the orbits; the presence of large muscle scars, bony knobs and other muscle attachment markers around the nasal opening (particularly in the dorsal region); retraction of the nasal bone (the 'roof' of of the nasal opening); deepening of the premaxillary bone (the bone making the jaw tip); anterior migration of the orbit; a large intraorbital canal (a foramen situated in the cheek region, just in front of the eye - it houses the nerves and blood vessels for our anterior face muscles); and strengthening of the posterior skull regions related to supporting the weight of the head on the neck (Wall 1980). Note that the criteria for elephant-like trunks are similar, but slightly different.

Paraceratherium transouralicum (formerly Baluchitherium grangeri) skull in dorsal, lateral and ventral views. Note features around the skull anterior linked to proboscis development (see text). From Osborn (1923b).

Paraceratherium skulls (above) meet these criteria well and, all else being equal, we have to say that yes, it looks likely that these giant rhinoceratoids had short proboscides in life, presumably to assist browsing from trees and bushes (Prothero 2013). The view that they had more typically rhinoceros-like faces is hard to defend in light of these cranial features: mammal skulls just don't have those retracted nasal openings, associated deep muscle scarring etc. unless they were doing something unusual and sophisticated with their upper lip and nasal tissues. The reality of giant indricotherines with dangly noses may seem hard to swallow for those of us used to shorter lipped versions, but given the relationships between rhinos and tapirs, the fact that some other fossil rhinocerotoids probably had proboscides as well (e.g. Wall 1980), and the independent development of long, flexible noses in numerous mammal lineages, we can't really see this as unusual. Moreover, we need to remember that modern rhinos are derived animals in their own right and separated from the indricotherine lineage by tens of millions of years. They aren't necessarily always going to be the best models for the life appearance of their fossil ancestors.

And big, elephant-like ears, right?

Finally, let's tackle the component that everyone now mentions about indricotheres since seeing the Carl Buell's cover art for Donald Prothero's Rhinoceros Giants:

Indiana University Press.

Yikes, elephant ears? For those of us familiar with the history of indricotheres in art, where their ears are restored as typically rhinoceros-like, this is a shocking, double-take image. Within the book, Prothero justifies the restoration:

"...indricotheres were larger in body mass than any living elephant and almost certainly had problems regulating their body heat at such large size. Elephants must do all they can to increase the surface area of their bodies to release as much excess heat as possible, which is why they have huge fan-like ears full of blood vessels that are essentially giant radiators. Given the huge size of indricotheres, it seems likely that they too should have had elephant-like ears, or at least very large ears of some shape, much larger than they are usually drawn."

The text continues to suggest that this appearance is not without anatomical support, the prominence of the mastoid and paroccipital processes (projections of bone situated behind the ear opening, adjacent to the posterior surface of the skull) being similar to the condition in certain elephants and mastodonts, and therefore indicative of large, flappy ears (Prothero 2013).

I have mixed feelings about this reconstruction. I like it for two reasons. The first is that it's nice to see indricotheres being distanced from their depiction as giant, long-necked rhinoceroses - again, it's not unreasonable to think they may have looked quite different in to modern rhinocerotids in many aspects. I also like these ears for being an All Yesterdays-style speculation on soft-tissue adaptations in extinct species. If we can use this as an excuse to give fat stores to desert sauropods or fuzzy hides to Arctic ceratopsids, then we can give large ears to giant rhinoceratoids.

On the other hand, I'm not convinced that they're as likely as Rhinoceros Giants suggests. It's clear from our modern fauna that ear size does not correlate with body mass in terrestrial mammals. By this logic many rhinos and giraffes should have proportionally large ears too, which they evidently do not. We also have to consider that even larger animals than indricotheres, dinosaurs, almost certainly got by without giant ears to help lose heat. And yes, while dinosaurs may have used different metabolic strategies to mammals, one inescapable consequence of giant size is a constant high body temperature. At least some investigations into the proportions of large dinosaurs suggest that development of their features - such as sauropod necks - were not driven by thermoregulatory pressures (Henderson 2013).

We should also consider the unusual nature of elephant thermoregulation: they are not typical mammals when it comes to controlling body heat. For one, they're atypically compact compared to other large mammals because they have extremely short necks, giant, round heads, and big, rotund torsos. This is a suboptimal bauplan for thermoregulation because it minimises surface area with respect to volume, and thus reduces the available area for elephants to dump excess heat. Moreover, unlike most mammals, they lack sweat glands (Wright and Luck 1984), do not pant, and they live in climates which are so warm that for much of the day they cannot shed heat through simple convection, big ears or not (Weissenböck et al. 2012). Elephants can, of course, regulate their temperature, but they need to employ different strategies to the rest of us mammals. These include maintaining moist skin with mud bathing and trunk spraying (Wright and Luck 1984), maintaining a sparse set of body hair to aid thermal escape (Myhrvold et al. 2012), using heterothermy (Weissenböck et al. 2012), the development of 'thermal windows' in their skin (Weissenböck et al. 2010), having loose and highly wrinkled skin to boost surface area and - of course - fanning their blood-vessel rich ears to help lose heat, when ambient temperatures are low enough for this to make a difference.

Silhouettes of the largest land mammals of all time, Paraceratherium transouralicum and Palaeoloxodon namadicus. Note the relatively gracile build of Paraceratherium - all the better for improving surface area:volume ratio, and thus superior for radiating heat. The numbers at the base of the image refer to estimated shoulder heights and tonnage. From Larramendi (2016).

These facts suggest elephants should not be used as direct thermoregulatory models for a giant rhinoceratoid. Modern rhinos other perissodactyls are much more typical in their thermoregulatory approaches: they have sweat glands and use panting behaviours (Hiley 1977) as well as some special tactics, such as enhanced vascularisation in the skin folds of certain rhino species (Endo et al. 2009). We have to assume that indricotherines at least had these entry level perissodactyl adaptations and, if so, they would have an advantage over elephants in hot climates. Indricotherines also benefit from being more complicated in form than elephants. They have longer limbs and necks, as well as a proportionally smaller head, and this enhances their surface area:volume ratio. Again, makes them better adapted to cope with heat as they have a shape better suited to radiating excess body heat. And of course, there's no reason to assume this could not have been augmented with wrinkled or folded skin or sparse hair. The picture emerging from these points is that big ears are only one strategy that big animals may use to keep cool, and maybe one that will only arise in specific circumstances. The idea that indricotherines would have big ears just because of their size is far from certain.

Basic muscle layout and trajectories (arrowed lines) of a modern horse. Note their superficial attachment and position high on the head - the ear canal itself is about halfway down the back of the skull. The 's' is the scutiform cartilage, which hangs out in front of the ear over the jaw muscles. From Goldfinger (2004).

But isn't all this moot because of Prothero's (2013) observations about the mastoid and paroccipital processeses being expanded, and thus giving big ears something to hang off? I'm suspicious about the significance of this observation. So far as I can determine, neither the mastoid or paroccipital have anything to do with anchoring ear tissues in modern perissodactyls (or perhaps any mammal). This might be because in most mammals - primates being one obvious exception - the ear pinnae are vertically displaced from the ear canal and attach to the head via a series of muscles and cartilages at the top of the skull (above). Only select few of the ear muscles reach the skull directly and these anchor, with very small attachments, to the skull midline, dorsoposterior margin and zygomatic arch. The rest have no osteological connection at all, anchoring instead to cartilage, membranes overlying facial musculature, or even the side of saliva glands. The paraoccipital and mastoid processes do have important roles in the muscular system but these are to do with neck, jaw and tongue muscles, not ears. Thus, unless indrictotheres were doing something different to modern mammals, those particularly big processes behind their ear openings were probably more to do with supporting and moving the head than they were holding big ears, and may have little significance to the big-eared indricotherine hypothesis.

So...

Putting all this together, it seems that there might be less need for uncertainty about indricothere appearance than our various artworks suggest. We should be saying 'yes' to some sort of proboscis, and 'probably not' to big ears (or, at least, 'there's no reason for them'). The elephant (or, giant rhino, if you prefer) in the room is the proportion issue, and it would be good to see folks who really know rhinocerotoid anatomy pore over those various reconstructions to ascertain which (if any) are the best representation of indricotherine form.

Next time: either the Next Big (but also kinda small) Thing in pterosaur research, or another trip to the Triassic.

Big rhinos need big support - thank goodness for Patreon

The paintings and words featured here are sponsored by another group of (metaphorically) giant mammals, my Patreon backers. Supporting my blog from $1 a month helps me produce researched and detailed articles with paintings to accompany them, and in return you get access to bonus blog content: additional commentary, in-progress sneak-previews of paintings, high-resolution artwork, and even free prints. For this post, we'll be taking a further look at the anatomy of the Paracertherium in my painting, above. Why do they have little manes and stripy faces? Are those child rhinos at the back a bit fuzzy? And why do the main animals look like they're fighting? Head over, and sign up to Patreon to get access to this and the rest of my exclusive content!

References

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