TYRANNOSAURS REX: THE NEW RESEARCH

Picture a Tyrannosaurus rex, that ferocious yet one of the most beloved dinosaurs. Most people will probably imagine a scaly giant with enormous fangs, visible even when its mouth is closed. This is the image of toothy predatory dinosaurs that popular culture has perpetuated for over 30 years. But our new study, published in Science, suggests that even the giant teeth of Tyrannosaurus would have been sheathed in scaly lips. Palaeontologists and artists have held different opinions on how dinosaur faces looked since we began recreating their form in the 1830s. From the 1980s onwards, artists and scientists have mostly shown theropod dinosaurs with lipless mouths and exposed teeth. No specific study or fossil discovery inspired this look. The widespread adoption of the lipless dinosaur reflected a preference for a new, ferocious-looking aesthetic rather than a scientific re-think.

This is not to say that lipless theropods are scientifically baseless. Living cousins of dinosaurs, the crocodylians (crocodiles and alligators), and the only surviving dinosaur group, birds, both have hard, immobile tissue around their jaws rather than the scaly lips of lizards. We found that predatory dinosaurs probably had lips like those you’d find on a lizard.

One part of our research looked at tooth damage. Exposed teeth show greater wear than those behind lips – for example, crocodylians have significant abrasion on their outer teeth. But when we examined theropod teeth using microscopes and compared them with crocodylian teeth, we found theropod teeth were considerably less damaged. This is not the only difference between theropods and crocodylians. All reptiles have small holes in their jaw bones that house blood vessels and nerves for their oral skin and gums, usually just millimetres wide. Lipped reptiles, lizards and tuataras (the last survivors of a group of lizard-like reptiles from the age of dinosaurs), have relatively few of these holes and they are mostly positioned close to their teeth. Crocodylian skulls, however, are covered in hundreds of tiny openings that are related to their sensitive, tight facial skin.

We found theropod jaw bones are more like lizards’ and have a low number of openings close to their jaw margins. This is also true of crocodylians’ ancient, extinct relatives. This implies that the unusual facial anatomy of living crocodylians evolved within their own lineage, not as a shared feature with the dinosaur/bird line.We also looked at tooth size, because some predatory dinosaurs had much bigger teeth than any living reptiles, and this might have prevented them from being enveloped inside lips. We calculated a ratio of tooth height and skull length for theropods. Then we compared this with the same value for monitor lizards, the lipped group that includes the only living animal comparable to large theropods in its feeding habits, the komodo dragon. Our comparisons revealed that no predatory dinosaurs – even the bigtoothed T. rex – had teeth larger than living lizards. Indeed, species like the crocodile monitor have proportionally larger teeth than any theropod, so there’s no reason to think dinosaur teeth were too big to be covered by lips. Finally, we modelled the mechanics of how lipless theropod jaws would close, and found it impossible for some theropods to seal their mouths without lips. The best we could manage was a gappy smile. Forcing jaws into a full seal either crushed jaw-supporting bones or dislocated the jaw joint. With permanently open mouths, these theropods would have faced issues with their oral health and risked dehydration.