Non-avian theropod dinosaurs such as the iconic Tyrannosaurus rex have long been portrayed with their teeth fully visible, similar to living crocodiles. This pattern of portrayal largely had to do with relatedness between dinosaurs and crocodiles and the relationship between tooth and jaw size. In new research, paleontologists from Canada, China and the United States tested hypothesized facial reconstruction in this group using analysis of tooth wear patterns and quantitative relationships between skull length and tooth size in both extinct and living reptiles. Contrary to depictions that have dominated for more than a century, they found that theropods, including Tyrannosaurus rex, had lips that covered their teeth, leaving them looking more like modern Komodo dragons than crocodiles.
With its mouth closed, all of the enormous teeth of Tyrannosaurus rex would be invisible behind its lips. Image credit: Mark Witton.
Non-avian theropod dinosaurs are renowned for their large, dagger-like teeth.
As a result, scientific and popular reconstructions of these dinosaurs have often featured these teeth as exposed, prominently protruding outside their closed mouths like crocodiles.
However, theropod teeth are known to have relatively thin enamel.
Since large theropod species likely retained their sharp and serrated teeth over long periods of time, it’s thought that constant exposure would likely lead to damaging tooth desiccation and wear.
Whether these ancient apex predators’ teeth were permanently exposed, as is often depicted, or covered by lip-like labial scales like a Komodo dragon, remains uncertain.
“Dinosaur artists have gone back and forth on lips since we started restoring dinosaurs during the 19th century, but lipless dinosaurs became more prominent in the 1980s and 1990s,” said Dr. Mark Witton, a paleontologist at the University of Portsmouth.
“They were then deeply rooted in popular culture through films and documentaries — Jurassic Park and its sequels, Walking with Dinosaurs and so on.”
“Curiously, there was never a dedicated study or discovery instigating this change and, to a large extent, it probably reflected preference for a new, ferocious-looking aesthetic rather than a shift in scientific thinking.”
“We’re upending this popular depiction by covering their teeth with lizard-like lips.”
“This means a lot of our favorite dinosaur depictions are incorrect, including the iconic Jurassic Park Tyrannosaurus rex.”
Comparisons of the reconstructions of Tyrannosaurus rex: (A) skull, based on Field Museum of Natural History specimen FMNH PR 2081; (B to E) two hypothetical flesh reconstructions, one with exposed teeth (B) and an associated cross section of the snout (C) and one with extraoral tissues covering the teeth (D) and an associated cross section of the snout (E). Image credit: Cullen et al., doi: 10.1126/science.abo7877.
To test hypotheses of theropod facial reconstructions, Dr. Witton and his colleagues evaluated the relationship between skull length and tooth size for a range of theropod dinosaurs and living and extinct toothed reptiles.
They performed a comparative histological analysis of tooth wear patterns for tyrannosaurid and crocodilian teeth.
In contrast to their closest toothed crocodilian relatives, theropod teeth lacked any evidence of outer surface wear, indicating the existence of extraoral tissues and oral secretions needed to keep them hydrated and protected from exposure.
What’s more, the paleontologists found that, even though the skulls and teeth of some theropods were far larger than extant reptiles, the tooth-skull size relationship in theropods closely aligned with that of living reptiles, particularly monitor lizards, who do not have exposed teeth.
The findings suggest that theropod teeth were not too big to fit in their mouth without having to be exposed.
“As any dentist will tell you, saliva is important for maintaining the health of your teeth,” said Dr. Kirstin Brink, a paleontologist at the University of Manitoba.
“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.”
“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.”
“Although it’s 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,” said Dr. Thomas Cullen, a paleontologist at Auburn University.
“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.”
“Some take the view that we’re clueless about the appearance of dinosaurs beyond basic features like the number of fingers and toes,” Dr. Witton said.
“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, we’re now at a point where we can say ‘oh, that doesn’t have lips? Or a certain type of scale or feather?’ Then that’s as realistic a depiction of that species as a tiger without stripes.”
The team’s paper appears in the journal Science.
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Thomas M. Cullen et al. 2023. Theropod dinosaur facial reconstruction and the importance of soft tissues in paleobiology. Science 379 (6639): 1348-1352; doi: 10.1126/science.abo7877
