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The teeth of Tyrannosaurus rex and other predatory dinosaurs would have been hidden behind scaly lips, which they couldn’t even pull back to bare their awesome fangs, contrary to film and TV depictions.

That is the conclusion of a team that has been studying the teeth of living reptiles such as alligators as well of those of ancient theropod dinosaurs, which include Velociraptor, Spinosaurus, tyrannosaurs and Giganotosaurus. “We need to update what the popular idea of a dinosaur is,” says at the University of Portsmouth, UK.

The case for a rethink put forward by Witton and his colleagues centres on tooth wear. Teeth that are always exposed should wear out more quickly than those kept inside the mouth.

This isn’t just to do with being physically shielded. If tooth enamel isn’t constantly kept wet by saliva, it dries out and becomes more brittle, says Witton.

When the team examined the exposed teeth of modern alligators, it revealed significant wear, with much of the enamel worn away. “It’s like someone took sandpaper to it,” says Witton.

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By contrast, the team found that fossil teeth from theropod dinosaurs were in really good condition, even though these dinosaurs didn’t replace their teeth as often as crocodilians do.

“That just isn’t consistent with them being outside the mouth like a crocodile,” says Witton.

The study also points out that many living predatory lizards have hidden teeth. For instance, the crocodile monitor lizard (Varanus salvadorii) has bigger teeth relative to its skull size than T. rex. Yet only the very tips of these teeth can be seen when its mouth is wide open, because the teeth aren’t only covered by its lips, but also sheathed in soft gum tissue that is pushed back by whatever it bites.

“In consequence, they actually bite into and cut their own gums sometimes,” says Witton.

“I suspect they 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,” says at the University of Edinburgh, UK.

There has been a lot of debate about tyrannosaur lips in recent years, says Brusatte, but it isn’t clear if T. rex was more like a crocodile or a lizard. “There is no good, modern-day analogue for a bus-sized superpredator with the head the size of a bathtub that crushed the bones of its prey.”

Witton doubts the study will persuade Hollywood to change its dinosaur depictions. He points out that film-makers have been reluctant to add feathers to screen dinosaurs even though the evidence that many had this feature is indisputable. So far, no T. rex fossils with feathers have been found, though.

“We’re decades behind the times in terms of portraying dinosaurs in cinema. We’re still in the age of Jurassic Park,” says Witton. “It’s getting a bit desperate at this point because they are so not reflecting what we think these animals looked like anymore.”