They're reptiles Jim, but not as we know them

A couple of Therizinosaurus cheloniformis in a Cretaceous woodland,  one of whom is making the loudest 'coo coo' noise you've ever heard, and one of whom couldn't care less.

There is no 'saur like a therizinosaur. That should be a well-known saying among palaeontologists, but it's not. They're all too busy doing science to come up with silly puns, no doubt. This painting of these magnificent animals came as a response to my previous post concerning 'Feather Resistance' (the fact that some people just don't like the idea of feathered dinosaurs) which featured a photo of the feathered remains of the small Chinese therizinosaur Beipiaosaurus. The neck vertebrae of this specimen thread through an extensive, posteriorly-expanding wedge of neck feathers in a fashion very reminiscent of diagrams of pigeon anatomy that we've all seen in textbooks, and it's fairly easy to see where my mind went from there (makes a change from all the cassowary-inspired paleoart on the internet, a trope which I'm guilty of myself). The composition of the image is something of an homage to the brilliant Crystal Palace sculptures constructed by Benjamin Waterhouse Hawkins between 1852 and 1855. Many species reconstructed by  Hawkins were shown with individuals in both reclining and standing poses, often with the standing animals looking rather more spectacular than those resting alongside. You can see this arrangement in several dioramas, including his famous Iguanodon, as well as his pterosaurs, Anoplotherium, Palaeotherium and Megaloceros (see below). I've always thought this enhanced the believability of Hawkin's work, showing his reconstructions as individual animals with distinct attitudes and personalities rather than species which simply engaged in one activity. Their poses are also frequently enhanced with the local topography, giving the impression the more active animals have ventured to higher ground to survey their surroundings or intimidate their audience. There's a lot to like about Hawkins' work and I think we may overlook him somewhat when considering the Great Palaeoartists, but that's a story for another day.

A gypsy-russel of Hawkins Crystal Palace sculptures, showing his flair for juxtaposed reclining and standing animals. Clockwise from left, Anoplotherium; Megaloceros (including bonus imagery of pterosaur researcher Michael O'Sullivan); Pterodactylus(?), and Iguanodon. If you've not visited these sculptures but are reading a blog like this one, you owe yourself a visit to Crystal Palce. Photos by Witton.

Behind the lousy wordplay attempted above lies an element of truth: is there a group of dinosaurs that undermines the classic concept of dinosaurs more than therizinosaurs, the feathered, pot-bellied, herbivorous theropods? Perhaps more than any other group, therizinosaurs highlight how much our understanding of dinosaurs has progressed not only since Hawkin's day, but even within the last few decades. Dinosaur concepts of the  1980s and 1990s stressed the more reptilian aspects of their nature, even in light of the undeniable birdiness of dromaeosaurs and other maniraptorans. Back then, dinosaur books and television shows presented lizards and crocodiles as the best modern analogues for dinosaurs, and birds were only mentioned as distant dinosaur relatives. We were told that dinosaurs weren't just reptiles in the taxonomic sense, but were reptilian in terms of their appearance and lifestyles, which echoed sentiments expressed as  long ago as the early 1800s. As we all know, early concepts of dinosaurs saw them as little more than gigantic super-reptiles, an idea best embodied by the models discussed above (and shown again, below), which were clearly primarily generated by wrapping modern reptiles around a few dinosaur fossils.

What's in a name?
Despite the advances in our concept of dinosaur palaeobiology since Hawkins' superlizards first roamed south London, we still use reptiles as modern points of reference for dinosaurs. If asked to concisely summarise Dinosauria in a single sentence for a lay audience, most of us would use the word 'reptile' somewhere. But, strange as it may seem to say, it doesn't really make much sense to introduce dinosaurs as 'reptiles' any more, and I think we do out of habit rather than good reason. Taxonomically speaking, the word 'reptile' is somewhat nebulous, with different definitions depending on its use. If you're old fashioned, you may define Reptilia as an artificial group containing all amniotes which are neither mammals or birds, but that doesn't work for dinosaurs as it excludes birds. Others would take the word 'reptile', as synonymous with 'Sauropsida', a natural group containing all amniotes except for the synapsids. That's fine, but Sauropsida is a big and diverse group, so labelling dinosaurs as 'reptiles' is an incredibly loose taxonomic address. It may have worked several decades ago, when concepts of reptile evolution were pretty murky, but not in today's world of cladograms and robust sauropsid phylogeny. Introducing dinosaurs as reptiles is correct, but not very informative. It's also inconsistent with the way we describe modern dinosaurs. We don't typically introduce birds as a group of reptiles: they're simply a type of animal. Why aren't other dinosaurs just described as 'animals' then? It seems that introducing dinosaurs as reptiles is either wrong, imprecise or inconsistent with the way we treat modern dinosaurs.

Hawkins' awesome model of a super-reptile Megalosaurus. Note the modern dinosaurs at the top of the photo, which give me an excuse to link to this. Photo by Witton.

Compounding this is the fact that, anatomically speaking, modern reptiles aren't a great match for dinosaurs, and there is an obviously superior alternative. Sure, many dinosaurs were scaly-skinned, terrestrial animals which laid eggs, so the 'reptile' reference still has some merit, but modern birds are far closer anatomical and behavioural analogues to Mesozoic dinosaurs. Birds also offer scaly skin, terrestrial habits and egg laying to dinosaurs, as well as filamentous integuments, rapid growth, extensive systems of air sacs in the body and neck, erect stances, long necks, and long limbs with digitigrade extremities (among many more detailed aspects of their anatomy). Because of this, we're all pretty happy that Mesozoic dinosaurs were a lot more like birds than lizards, snakes or crocodiles, but we still preferentially mention reptiles rather than birds when introducing the group.

So why is the word 'reptile' at the heart of our basic concept of Mesozoic dinosaurs, when using birds as an immediate point of reference would make more sense both anatomically and taxonomically? Maybe it's simply because the word 'reptile' got there first. The reptilian nature of dinosaur fossils was appreciated before their birdiness, so we labelled them 'fossil reptiles'. And it stuck. We probably would not be in this position if the first dinosaur discoveries had been of deinonychosaurs, oviraptorosaurs or another, obviously bird-like species, rather than fragments of megalosaurs or ornithischians. What if evidence of feathered non-avian dinosaurs had been available to early dinosaur workers? It could have happened that way. Feather impressions are now known to occur in multiple fossil sites around the world, including some deposits which may not, at first, seem likely to preserve them. Even in sites where feathers do not preserve, the forelimbs of some dromaeosaurs and ornithomimids preserve feather-anchoring quill knobs. It's not inconceivable, therefore, that the birdiness of dinosaurs could have been revealed much earlier on in dinosaur research history, and that the concept of dinosaurs as fossil reptiles may never have been established. Perhaps, given how essential the reptilian nature of dinosaurs seems to their popularity, dinosaurs would never have become anywhere near as popular if this version of history had played out. Just think: we may never have had Jurassic Park.

So there we have it, then: introducing dinosaurs as reptiles is a bit silly but, like a lot of language, we stick with it because of its established nature and ease of use. With hundreds of years of momentum behind this idiosyncrasy, I doubt we'll ever see dinosaurs labelled as anything else. However, that's not to say that educators or science communicators wouldn't benefit from occasionally tweaking the way that they introduce Mesozoic dinosaurs to their audiences. Perhaps replacing the word 'reptiles' with 'bird-relatives' every now and then will jar a few minds awake, and especially if said educators are trying to put some distance between modern dinosaur concepts and those of the past.