The Tyrannosaurus rex is often shown baring massive, sharp teeth, like the ferocious creature in Jurassic Park.
But new research suggests that this classic image might be wrong.
The teeth on T-rex and other big theropods were probably covered by scaly lips, concludes a study published on Thursday in the journal Science.
The dinosaur’s teeth did not stick out when its mouth was closed, and even in a wide open bite, you might just see the tips, the scientists found.
The research is the latest in a long back-and-forth over how dinosaur mouths really looked.
Recent depictions show big teeth jutting out of the dinosaurs’ jaws, even when closed.
Some thought the predators’ teeth were just too big to fit in their mouths, said study author Thomas Cullen, a palaeontologist at Auburn University in Alabama.
When researchers compared skulls from dinosaurs and living reptiles, though, they found this was not the case.
Some large monitor lizards actually have bigger teeth than T-rex compared to their skull size, and can still fit them under a set of scaly lips, Mr Cullen said.
The scientists also found clues in the pattern of wear and tear on tooth surfaces.
For a creature like a crocodile, whose teeth stick out of its mouth, the exposed part gets worn down quickly, “like someone’s taken a sander to the side of the tooth", said another study author Mark Witton, a palaeo-artist at England’s University of Portsmouth.
But when researchers analysed a tooth from a Daspletosaurus, a T-rex relative, they found it was in good condition and did not show that uneven damage pattern.
With this evidence and other clues from the dinosaurs’ anatomy, the study makes a good case for lipped tyrannosaurs, said University of Maryland palaeontologist Thomas Holtz, who was not involved with the study.
“We’re not talking kissy lips,” Mr Holtz said.
They would be thin and scaly like those of the Komodo dragon, a large lizard.
It is not the first time our depictions of dinosaurs have been called into question.
Other research has shown that T-rex was more hunched over than we thought, and that fierce velociraptors probably had feathers.
Most of what we know about dinosaurs comes from their bones, but it can be harder to get clear answers about soft tissues like skin, which usually are not preserved in fossils.
Adding lips may make dinosaurs look a little less ferocious, but it also makes them feel more realistic, Mr Witton said.
“You don’t really see a monster,” he said. “You see an animal.”