Jurassic Makeover: Reveal the true faces of predatory dinosaurs like T. rex

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Researchers and artists have debated whether theropod dinosaurs, the group of two-legged dinosaurs that includes carnivores and top predators likeT. rexandVelociraptor, as well as birds, had lipless mouths where perpetually visible upper teeth hung over their lower jaws, similar to the mouth of a crocodile.

T. rex skull and head reconstructions. Credit: Mark Witton

However, an international team of researchers challenge some of the best-known depictions, and say these dinosaurs had lips similar to those of lizards and their relative, the tuatara a rare reptile found only in New Zealand, which are the last survivors of an order of reptiles that thrived in the age of the dinosaurs.

In the most detailed study of this issue yet, the researchers examined the tooth structure, wear patterns and jaw morphology of lipped and lipless reptile groups and found that theropod mouth anatomy and functionality resembles that of lizards more than crocodiles. This implies lizard-like oral tissues, including scaly lips covering their teeth.

These lips were probably not muscular, like they are in mammals. Most reptile lips cover their teeth but cannot be moved independently they cannot be curled back into a snarl, or make other sorts of movements we associate with lips in humans or other mammals.

Study co-author Derek Larson, Collections Manager and Researcher in Palaeontology at theRoyal BC Museum in Canada, said: Palaeontologists often like to compare extinct animals to their closest living relatives, but in the case of dinosaurs, their closest relatives have been evolutionarily distinct for hundreds of millions of years and today are incredibly specialised.

Its quite remarkable how similar theropod teeth are to monitor lizards. From the smallest dwarf monitor to the Komodo dragon, the teeth function in much the same way. So, monitors can be compared quite favourably with extinct animals like theropod dinosaurs based on this similarity of function, even though they are not closely related.

Tyrannosaurus rex bellowing with its mouth shut, like a vocalising alligator. With its mouth closed, all of the enormous teeth of T. rex would be invisible behind its lips. Credit: Mark Witton

Co-author Dr Mark Witton from the

A half-grown Tyrannosaurus, sporting a full set of lips, runs down Struthiomimus, a beaked ostrich dinosaur. Credit: Mark Witton

Also, the distribution of small holes around the jaws, which supply nerves and blood to the gums and tissues around the mouth, were more lizard-like in dinosaurs than crocodile-like. Furthermore, modelling mouth closure of lipless theropod jaws showed that the lower jaw either had to crush jaw-supporting bones or disarticulate the jaw joint to seal the mouth.

As any dentist will tell you, saliva is important for maintaining the health of your teeth. Teeth that are not covered by lips risk drying out and can be subject to more damage during feeding or fighting, as we see in crocodiles, but not in dinosaurs, said co-author Kirstin Brink, Assistant Professor of Palaeontology at the University of Manitoba.

She added: Dinosaur teeth have very thin enamel and mammal teeth have thick enamel (with some exceptions). Crocodile enamel is a bit thicker than dinosaur enamel, but not as thick as mammalian enamel. There are some mammal groups that do have exposed enamel, but their enamel is modified to withstand exposure.

A one-sheet summary of the main investigations and conclusions of the study. Credit: Mark Witton

Thomas Cullen, Assistant Professor of Paleobiology at Auburn University and study lead author, said: Although its been argued in the past that the teeth of predatory dinosaurs might be too big to be covered by lips, our study shows that, in actuality, their teeth were not atypically large. Even the giant teeth of tyrannosaurs are proportionally similar in size to those of living predatory lizards when compared for skull size, rejecting the idea that their teeth were too big to cover with lips.

The results provide new insights into how we reconstruct the soft tissues and appearance of dinosaurs and other extinct species. This can give crucial information on how they fed, how they maintained their dental health, and the broader patterns of their evolution and ecology.

Dr Witton said: Some take the view that were clueless about the appearance of dinosaurs beyond basic features like the number of fingers and toes. But our study, and others like it, show that we have an increasingly good handle on many aspects of dinosaur appearance. Far from being clueless, were now at a point where we can say oh, that doesnt have lips? Or a certain type of scale or feather? Then thats as realistic a depiction of that species as a tiger without stripes.

The researchers point out that their study doesnt say that no extinct animals had exposed teeth some, like sabre-toothed carnivorous mammals, or marine reptiles and flying reptiles with extremely long, interlocking teeth, almost certainly did.

For more on this research, see Deadly Teeth of T. rex Were Hidden Behind Scaly Lips.

Reference: Theropod dinosaur facial reconstruction and the importance of soft tissues in paleobiology by Thomas M. Cullen, Derek W. Larson, Mark P. Witton, Diane Scott, Tea Maho, Kirstin S. Brink, David C. Evans and Robert Reisz, 30 March 2023, Science.
DOI: 10.1126/science.abo7877