Brimming with serrated teeth larger than bananas, the fanged maw of the Tyrannosaurus rex is iconic. Many depictions of the prehistoric predator show its teeth protruding even when its mouth is closed, like a crocodile with crooked teeth.
However, some paleontologists believe that T. rexes need some serious lip filler. In a study published Thursday in Science, researchers postulate that tyrannosaurs and related dinosaurs kept their dagger-shaped dentition hidden behind lip-like tissue. And while some critics are joking about it, the study authors say it’s time we reconsidered what the apex dinosaur’s mouth looked like as it tore at prey.
While birds are the closest living relatives to theropod dinosaurs — the group that included mega-predators like T. rex — their specialized beaks tell scientists little about dinosaur mouths. Therefore, researchers have often turned to crocodiles, whose exposed teeth protrude directly from the jaw with no lip-like tissue covering them. Even when a crocodile’s jaws are closed, its teeth are visible.
This has led many scientists and artists to depict lipless dinosaurs with their teethers constantly in view. One of the most influential depictions is the Tyrannosaurus from “Jurassic Park.”
“That animal has been copied so many times,” said Mark Witton, a paleoartist and researcher at the University of Portsmouth in England, who has been illustrating lipped theropods for about a decade. “It brought that lipless look into pop culture to the point where we now struggle to get rid of it.”
Dr. Witton is among the scientists and paleoartists who have argued that theropods had a fleshy area around their mouths. He and other colleagues recently teamed up to search for fossil evidence of these fuller lips.
They focused on the teeth. According to the researchers, the teeth of many theropods were covered by a thin layer of enamel. The researchers hypothesized that constant exposure to air could cause the enamel to become brittle and prone to chipping. Lipless crocodiles, for example, wear down their teeth at an accelerated rate: an American alligator can have 3,000 teeth in its lifetime. By contrast, tyrannosaurs and other theropods tended to hold on to their teeth much longer.
To compare wear patterns between crocodiles and theropods, the team studied thin cross sections of teeth from an American alligator and Daspletosaurus, a close relative of T. rex. They found that the enamel on the exposed outer side of the alligator tooth was often more eroded than the inner part of the tooth.
“We don’t see that pattern at all in tyrannosaurs,” said Thomas Cullen, a paleontologist at Auburn University and one of the paper’s authors.
The wear on the Daspletosaurus tooth was different, a sign, they said, that a lip-like covering protected it from drying out.
“In our Tyrannosaur sample, we see a uniform thickness of 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 monitor lizards. These reptiles have blade-shaped teeth reminiscent of theropod teeth, which they keep moist under their scaly lips. While monitor lizards are only distantly related to theropods, the team found that the relationship between skull size and teeth was similar. That resemblance dispels any notion that larger carnivorous dinosaurs would have trouble fitting their teeth under their lips, they said.
But not all paleontologists are convinced by the theropod lips.
Thomas Carr, a paleontologist at Carthage College in Wisconsin, said the researchers did not take into account the bone texture of the T. rex skulls, which resembled the leathery texture of an alligator skull up to where the teeth are embedded in the jaw. He also argued that the dentin in tyrannosaurus teeth was more important than the enamel. “That’s the tissue that I think is structurally most important to a T-rex because if the dentin breaks down, then they’ll eat bananas,” said Dr. Carr. As a result, he believes that keeping the enamel under the lips moist was not essential. to keep teeth strong enough to bite through bone.
The only thing that could bring the dinosaur lip debate to extinction could be a fossilized face. “We won’t have a firm answer unless we find a really rare example of a theropod with intact facial soft tissues preserved,” Dr. Cullen said. “It’s not impossible, it just hasn’t happened yet.”