To test the hypothesis that lips covered theropod dinosaurs’ teeth when the mouth was closed, Reisz and colleague Derek Larson examined dental anatomy and modern-day reptilian predators, including crocodiles and monitor lizards.

Crocodiles have exposed teeth, much like a “Jurassic Park” predator, while monitor lizards have teeth concealed behind scaly lips, similar to the movie version of velociraptors, he said.

Lips protect teeth enamel from drying out in part by keeping them hydrated, Reisz said.

“We propose that this requirement of hydration is not possible to maintain if the tooth is exposed permanently,” he said.