The scaly lips of the Tyrannosaurus rex
In almost all illustrations, the predatory dinosaurs show teeth even when their mouths are closed. However, the reality was different: The dinosaurs probably had lips that covered their teeth.
When Tyrannosaurus rex closed its mouth, its teeth were no longer visible - thin, scaly lips completely covered the teeth. This is the conclusion reached by a research group led by Robert Reisz from the University of Toronto Mississauga on the basis of dental examinations and anatomical comparisons of theropods with monitor lizards and crocodile relatives. Crocodile teeth are clearly visible even when the mouth is closed, and predatory dinosaurs are usually also depicted with protruding teeth - monitor lizards, on the other hand, have lips that cover the teeth. As the team now reports in "Science", the proportions of the predatory dinosaur teeth are more similar to those of monitor lizards than crocodiles, despite their often enormous size. In addition, the dinosaur teeth lack the damage that often occurs in crocodiles due to their position outside the mouth.
The research group thus clarifies an old question of dinosaur anatomy. After all, soft tissue such as the thin lips are only very rarely preserved as fossils, and no fossilised dinosaur lip has ever been found. In popular and scientific illustrations, however, the matter seemed clear for a long time: even with the mouth closed, the teeth of the upper jaw can be seen outside the mouth in most pictures. The new study has now put an end to this prehistoric overbite. According to Reisz and his team, there are several indications pointing to the existence of lips in predatory dinosaurs. For example, the argument often put forward against this theory is not valid: although the teeth are powerful, they are no larger in comparison to the skull than those of the lipped monitor lizards - lips could therefore also have protected the banana-sized jaws of a tyrannosaur.
The teeth themselves also show that this must have been the case. If you don't have lips, it's quite difficult to keep your teeth permanently moist. Tooth enamel contains a small amount of water and can therefore dry out. Although dry enamel is harder, it is also more sensitive to scratches and abrasion - and without lips, it is easy to knock your teeth against something. This is why crocodile teeth show significantly more wear on the outside than on the inside, whereas the teeth of predatory dinosaurs do not. This proves that the soft tissue outside the mouth is more similar to modern scaly lizards and other quadrupeds than crocodiles or birds, writes the working group. However, future "Jurassic Park" instalments will probably not feature any smooching velociraptors: Unlike mammals, whose lips are mobile and studded with sensory cells, dinosaurs probably only had a kind of protective tissue in front of their teeth.
Spectrum of Science
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Cover image: © Mark P. Witton 2022 (detail) A tyrannosaur whose impressive teeth are covered by lips looks a lot more relaxed and likeable. But that's deceptive.
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