Imagine Tyrannosaurus rex, the wild one, but one of the most popular dinosaurs. Most people probably imagine a scaly giant with huge fangs that are visible even when his mouth is closed. Predatory dinosaurs that popular culture has perpetuated for over 30 years. Tyrannosaurus’ giant teeth would have been covered by scaly lips.
Paleontologists and artists have held different views on what dinosaur faces looked like since we began recreating their likeness in the 1830s. Since the 1980s, artists and scientists have mostly shown theropod dinosaurs (a line that includes Tyrannosaurus, Velociraptor, and birds) with lipless mouths and bared teeth.
This look became deeply ingrained in popular culture thanks to the 1993 film Jurassic Park and its iconic depiction of the T. rex. The creators of Jurassic Park deliberately exaggerated the size and visibility of their tyrant’s teeth, despite the fact that at the time it was an otherwise accurate representation of Tyrannosaurus.
Tyrannosaurus’ giant teeth would have been covered by scaly lips
This is not to say that lipless theropods are scientifically unfounded. Living relatives of the dinosaurs, the crocodilians (crocodiles and alligators) and the only surviving group of dinosaurs, the birds, both have hard, immovable tissue around their jaws rather than the scaly lips of lizards. So it was reasonable to infer that extinct animals related to crocodiles and birds (including all predatory dinosaurs) had faces without lips.
One part of our research looked at damage to teeth. Exposed teeth show more wear than labial teeth – for example, crocodiles have significant abrasion on their outer teeth. But when we examined theropod teeth with microscopes and compared them to crocodile teeth, we found that theropod teeth were significantly less damaged.
This is not the only difference between theropods and crocodiles. All reptiles have small openings in their jawbones that contain blood vessels and nerves for the skin in the mouth and gums, usually only millimeters wide.
Lipid reptiles, lizards and tuatars (the last survivors of the lizard-like reptile group from the time of the dinosaurs) have relatively few of these holes and are mostly located near the teeth. However, crocodile skulls are covered with hundreds of tiny holes that are related to their sensitive, taut facial skin.
Theropod jawbones
We found that the theropod jawbones are more like lizards and have a small number of holes near the edges of the jaws. This also applies to ancient, extinct relatives of crocodiles. This means that the unusual facial anatomy of living crocodilians evolved within their own lineage, not as a shared trait with the dinosaur/bird lineage.
We also looked at tooth size, because some predatory dinosaurs had teeth much larger than other living reptiles, and this may have prevented them from wrapping inside their lips. We calculated the ratio of tooth height to skull length for theropods.
Our comparisons revealed that no predatory dinosaurs not even the big-toothed T. rex—had teeth larger than living lizards. Species like the crocodile monitor have proportionally larger teeth than any theropod, so there’s no reason to think that dinosaurs’ teeth were too big to cover their lips.
Finally, we modeled the mechanics of how lipless theropod jaws would close and found that it is impossible for some theropods to seal their mouths without lips. The best we could manage was an embarrassed smile. Pushing the jaws into a full seal either crushed the bones supporting the jaw or dislocated the jaw joint. With their mouths permanently open, these theropods would have faced oral health problems and risked dehydration.
Combined with the scaly lips, our findings suggest that predatory dinosaurs may have had softer faces and mouths than we’re used to. But don’t let that fool you. Behind those lips and gums were the same formidable, flesh-ripping teeth.
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