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Dinosaurs may have had scaly 'lips' to protect their teeth from damage
Carnivorous dinosaurs may have had lips to hide their teeth behind.
Rather than keeping them permanently exposed like crocodiles, a new study suggests that the teeth of the two-legged theropods would have been more similar to monitor lizards.
Tyrannosaurs may not have been able to flash a toothy grin at their fellow dinosaurs.
While iconic depictions of Tyrannosaurus rex in films such as Jurassic Park show the points of its teeth jutting out of its mouth, a new paper argues this might not have been the case. A relative lack of tooth wear compared to living reptiles instead suggests that they were covered by scaly 'lips'.
Dr Mark Witton, a co-author on the paper and noted palaeoartist, says, 'Dinosaur artists have gone back and forth on lips since the nineteenth century, but lipless dinosaurs became more prominent in the 1980s and 1990s. They were then deeply rooted in popular culture through films and documentaries such as Jurassic Park and Walking with Dinosaurs.'
'Curiously, there was never a dedicated study or discovery instigating this change and, to a large extent, it probably reflected a 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, meaning that many of our favourite dinosaur depictions are incorrect.'
Unlike mammals, these lips probably didn't contain muscles that would have allowed them to be moved for expressions such as snarling. Instead, the researchers believe the lips would have acted as a cover to protect the teeth from wear and tear.
The findings of the study were published in the journal Science.
Alligators have external teeth, but as a result they are much more worn than teeth from lipped reptiles. Image © chloe7992/Shutterstock.
Why might dinosaurs have had lips?
While bones fossilise relatively easily, soft tissues are only preserved in very rare circumstances. This means that it's relatively simple to work out the rough shape of an animal from its skeleton, but depicting its appearance is much more difficult.
For the vast majority of fossils without soft tissue preservation, how an animal looks has to be inferred from other sources of evidence. In fossilised feathers, for instance, the shape of structures known as melanosomes can be used to work out how colourful the animal was.
Researchers also looked at relatives of extinct species to give hints about how they could have looked. As the closest relatives of dinosaurs, the birds, have beaks rather than teeth, this new study had to look at more distant relatives.
Derek Larson, a palaeontologist at the Royal BC Museum, says, 'While they're not closely related, it's 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.'
'This means that 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.'
The team compared a tooth from Daspletosaurus, a relative of Tyrannosaurus rex, to teeth from modern lizards which have lips as well as an alligator and extinct relative of modern crocodiles, which don't. They found that the structure of the dinosaur tooth was different from those of crocodilians, which tend to have thicker enamel.
Lips also help to protect the enamel from wear by keeping it moist. Sections of the alligator tooth that sat outside the mouth were much more worn than those of Daspletosaurus, adding further evidence that theropod dinosaurs may have had outer lips.
According to the study, the lips of theropod dinosaurs were probably like those of monitor lizards. Image © Martin Pelanek/Shutterstock.
While they may have had lips, there is always the possibility that the teeth were simply too big to fit into them. To examine this in more detail, the team compared the relationship between skull and tooth size in lipped monitor lizards with that of the theropods, finding that they followed a broadly similar trend.
Dr Thomas Cullen, Assistant Professor of Paleobiology at Auburn University and lead author of the study, says, '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 their teeth were not atypically large.'
'Even the giant teeth of tyrannosaurs are proportionally similar in size to those of living monitor lizards when compared for skull size, rejecting the idea that their teeth were too big to cover.'
The team have even suggested that the ancestor of all dinosaurs may have been lipped, though there's currently not enough evidence to prove this either way.
Finding out about the lips of dinosaurs will help scientists to better understand what these animals looked like, and how they might have eaten over 66 million years ago.
