While T rex is often depicted prowling the landscape while sporting a toothy grin, its fearsome teeth may actually have been concealed behind a pair of thin, scaly lips, research suggests.
Experts say the perception that theropods were lipless arose because of the huge size of their teeth, and because their closest living toothed relatives – such as crocodiles and alligators – do not have lips.
However, the research suggests that, like lizards today, theropods may have had their teeth covered when their mouth was closed.
Dr Mark Witton, of the University of Portsmouth and a co-author of the study, said popular depictions of dinosaurs were out of date.
“We are basically still living in the shadow of Jurassic Park [from] 30 years ago,” he said. “We need to move away from this toothy lipless look for things like Tyrannosaurus and towards these animals having more lizard-like faces.”
Writing in the journal Science, researchers in the US and UK say the examination of a large tooth that spent over 500 days in the mouth of the tyrannosaur Daspletosaurus revealed no evidence of substantial wear – a finding consistent with studies of other theropod teeth.
By contrast, the large teeth of American alligators are often damaged, with even the dentine layer worn down – a result, the researchers suggest, of the animals being lipless, meaning their tooth enamel is exposed, which is why it becomes dry and less resistant to wear.
Witton said tyrannosaur teeth often lasted more than 12 months before being replaced – far longer than for crocodiles – adding weight to the idea the former had lips.
“No animals can repair or replace worn enamel, and yet the thin enamel of tyrannosaurs remains intact even though some retained their teeth well over a year,” he said.
The team also found tiny holes in the jaws of theropods arranged in a similar way to those in today’s lizards – where they supply the lips and the gums with nerves and blood vessels – while both have vertical teeth, unlike crocodiles whose teeth lean outwards.
The team said an analysis of the relative size of the skull and teeth in lizards today suggested theropod teeth were not too large to have been covered by lips.
“If you just imagine the Komodo dragon scaled up with a 5ft-long skull, it is not going to look much different from something like a T rex,” said Witton.
Prof Steve Brusatte, of the University of Edinburgh, who was not involved in the work, said that if the researchers were correct, T rex would not have had a toothy grin but more of a gummy smile, adding that the soft tissue covering their teeth would not be the same shape as our own fleshy and pouty lips.
However, Brusatte said the case was not yet closed. “I suspect [these researchers] are right, and that tyrannosaurs had more soft tissue covering their teeth than crocodiles, but I’m still on the fence as to whether they had as much stuff covering their teeth as monitor lizards,” he said.