T. rex may have had lips. Yes, you read that right. Lips.
Robert Reisz, a paleontologist at the University of Toronto, is challenging the long-standing image of meat-eating theropod dinosaurs such as T. rex. Specifically, Reisz suggests that theropods' teeth were not bared all the time, extending outside their mouths and fully visible whether their jaws were open or closed. Rather, these teeth were kept hidden, covered by scaly lips, he said in a presentation May 20 at the Canadian Society of Vertebrate Paleontology's annual meeting in Ontario. [Gory Guts: Photos of a T. Rex Autopsy]
Reisz told Live Science in an email that he had always been bothered by the typical "permanent smile" portrayal of theropod dinosaur teeth. He first looked to the closest living relatives of theropod dinosaurs - crocodiles - for clues about tooth exposure.
RELATED: T. Rex Was Likely an Invasive Species
At first glance, it could seem like the expectation for large theropods to have exposed teeth was on the right track. Crocodiles' teeth are covered by gums for about one-quarter of their length, but lips are absent and the tooth crowns are permanently exposed, Reisz explained.
However, if you look closer at tooth structure, a different story might emerge, he noted in his presentation.
The hard enamel of animals' teeth has low water content, and is typically kept hydrated by saliva. Without lips to keep moisture in and prevent the teeth from drying out, the tough enamel would become brittle and more prone to damage and wear, Reisz told Live Science.
RELATED: This T. rex-Like Dino Was Vegetarian
Crocodiles live in watery environments and would rely on their habitat to keep exposed teeth hydrated. But land-dwelling theropods' large teeth - which are known to have enamel - could have been compromised by perpetual exposure, and likely needed to be covered by lips in order to stay moist, Reisz said in the presentation.
But crocodiles aren't the only animals with exposed teeth - elephants, for instance, have exposed teeth as well, and many extinct saber-toothed predators had very long canines that were also exposed when their mouths were closed. Wouldn't their teeth have been vulnerable to serious drying out, too?
Not necessarily. A mammal's tooth structure is actually quite different from a reptile's, said Zhijie Jack Tseng, a paleontologist who studies bite-force biomechanics in extinct carnivores at the American Museum of Natural History in New York City.
WATCH VIDEO: Could a Fossilized Mosquito Resurrect Dinosaurs?