Imagine a T-Rex. Now imagine that with the lips.

Filled with serrated teeth larger than a banana, the fanged Mau of the Tyrannosaurus Rex is iconic. Many images of the prehistoric predator show its teeth protruding even when its mouth is closed, like the saber-toothed crocodile.

However, some paleontologists think T. rexes needed some serious lip fillers. In a study published Thursday in SciencesResearchers hypothesize that tyrannosaurs and related dinosaurs kept their dagger-like teeth hidden behind lip-like tissue. And while some critics are vocal about this, the study’s authors say it’s time to rethink what the primate dinosaur’s mouth looked like as it ripped open its prey.

While birds are the closest living relatives of theropod dinosaurs, a group that included massive predators like T. rex, their specialized beaks tell scientists quite a bit about dinosaur mouths. So researchers have often turned to crocodiles, whose exposed teeth protrude directly from the jaw without being covered by lip-like tissue. Even when the crocodile’s jaw is closed, its teeth are visible.

This has led many scientists and artists to depict lipless dinosaurs with their silos constantly on display. One of the most influential images is the Tyrannosaurus from “Jurassic Park”.

“This animal has been copied many times,” said Mark Whitton, a paleoanthropologist and researcher at the University of Portsmouth in England who has been photographing the labial theropod for about a decade. “It carried that lipless look into pop culture to the point where we’re now struggling to get rid of it.”

Dr. Wheaton is among the ancient scientists and artists who have argued that theropods had a fleshy region surrounding their mouths. He and other colleagues recently teamed up to search for fossil evidence of these plump lips.

Focus on the teeth. According to the researchers, many theropod teeth were coated with a thin layer of enamel. The researchers hypothesized that constant exposure to air could make the enamel brittle and prone to splitting. Lipless crocodilians, for example, wear teeth at an accelerated pace—an American alligator can go through 3,000 teeth in its lifetime. In contrast, tyrannosaurs and other theropods tended to hold their teeth longer.

To compare wear patterns between crocodilians and theropods, the team studied thin cross-sections of the teeth of an American alligator and Daspletosaurus, a relative of Tyrannosaurus rex. They found that the enamel on the outer, exposed side of alligator teeth was often more eroded than the inner part of the tooth.

“We don’t see this pattern at all in tyrannosaurs,” said Thomas Cullen, an Auburn University paleontologist and one of the paper’s authors.

The wear on Daspletosaurus’ teeth was different, and they said a lip-like covering protected it from drying out.

“In our Tyrannosaur specimen, we see an even thickness of the enamel on both the inside and outside of the tooth which is more similar to what we see in animals that have lips,” said Dr. Cullen.

The team also examined the skulls of Komodo dragons and other lizards. These reptiles have blade-like teeth reminiscent of those of theropods, which they keep moist under scaled lips. While monitor lizards are closely related to theropods, the team found that the relationship between skull and tooth size was similar. The similarity, they said, dispels any notion that the largest carnivorous dinosaurs would have had difficulty securing their teeth under their lips.

But not all paleontologists are sold on theropod lips.

Thomas Carr, a paleontologist at Carthage College in Wisconsin, said the researchers did not take into account the bone tissue of Tyrannosaurus skulls, which is similar to the leathery tissue of a crocodile’s skull down to where the teeth penetrate the jaw. He also argued that the dentin of Tyrannosaurus teeth was more important than enamel. “This is the tissue that I think structurally is most important for a dinosaur because if the ivory breaks down, they will eat bananas,” said Dr. Carr. As a result, it is believed that keeping the enamel moist under the lips was not necessary to keep the teeth strong enough to bite through bone.

The only thing that could drive the dinosaur lips debate to extinction might be a fossilized face. “We won’t have a firm answer unless we find a really rare example of a theropod with preserved facial soft tissues intact,” said Dr. Cullen. “It’s not impossible – it just hasn’t happened yet.”