The T. rex's large teeth are its most scary feature (Credit: Public Domain)

The Tyrannosaurus rex (T. rex) has always been portrayed with its razor-sharp teeth jutting out of its jaw. They are visible even when its mouth is closed. The depiction is based on its closest relative - the crocodile. Also, scientists assumed that the dinosaur's teeth, which grew up to six inches (15.2 cm) long, were too large to fit inside its mouth.

But Auburn University paleontologist Thomas Cullen and his team assert that the T. rex and other theropods had scaly, lizard-like lips that covered their massive teeth. If true, the T. rex may not have looked as frightening as we had imagined.

The extensive study was published in the journal Science on March 30, 2023. It compared the skull lengths and tooth sizes of over 20 fossilized theropods. The scientists also examined the bones of lizards, like the Komodo dragon. In each case, they found that the skulls were big enough to fit the teeth even when the mouth was closed. Also, the tiny holes that supplied nerves and blood to the gums and tissues in theropods resembled those found in lizards, not crocodiles.

Some researchers believe that the T. rex and other theropods had lips that hid their large teeth (Credit: Mark Witton/ University of Portsmouth)

To confirm their theory, the scientists analyzed a Daspletosaurus tooth. The theropod is considered the T. rex's direct ancestor. They found that the enamel was almost intact. This would not have been the case if the tooth had been exposed to dry air constantly. The researchers also argue that crocodiles go through as many as 3,000 teeth during their lifetime because they are not protected by lips. However, the T. rex required two years to replace just one tooth. They believe this is further proof that the dinosaurs had lips. The scientists suspect the lips were similar to those found in other reptiles. They covered the teeth but lacked the muscles to move independently.

Cullen says, "Although it's been argued in the past that the teeth of predatory dinosaurs might be too big to be covered by lips, our study shows that, in actuality, their teeth were not atypically large. Even the giant teeth of tyrannosaurs are proportionally similar in size to those of living predatory lizards when compared for skull size, rejecting the idea that their teeth were too big to cover with lips."

A rendering of a T. rex with lips (Credit: Mark Witton/ University of Portsmouth)

But Thomas Carr, a paleontologist at Carthage College in Wisconsin, is unconvinced. In 2017, Carr and his team found that the facial bones of theropods resembled those of crocodiles. This led them to conclude that their mouths were scaly and had no lips.

Cullen believes the debate will continue until someone finds a theropod fossil with intact skin. The scientist says, "We won't have a firm answer unless we find a really rare example of a theropod with soft tissues of the face preserved intact. It is not impossible — it just hasn't happened yet."