T-Rex had lips

Dinosaurs exist in movies, books, museums and computer games. And we’ve interpreted what we think they looked and sounded like based on their fossilised remains, which inevitably means that a lot of guesswork has been involved. And if you picture a stereotypical T Rex, you probably end up imaging a fearsome-looking array of exposed teeth projecting from both jaws. But we might have missed a crucial detail about the faces of these animals: they probably had lips that would have hidden those teeth! When teeth are covered by lips they stay wet, and this protects the enamel. And by looking at fossilised teeth from T Rex type dinosaurs, Auburn University’s Thomas Cullen has found that they don’t have the characteristic external wear you see in animals with exposed teeth, like alligators. So it looks like it’s time to redraw the text book depiction of T Rex - with lips!

Thomas - We're all really interested in the way that people publicly perceive dinosaurs and when people are building up reconstructions, what these animals look like, they will often need to make sort of guesses or estimations about things we don't have direct fossil evidence of. And in this case, lips or soft tissues around the mouth that could cover the teeth or their absence was the area of interest here. And in the past people have reconstructed a lot of dinosaurs to look sort of like crocodiles with no lips. And we were curious if there was any actual evidence to support that and so we decided to try and investigate it.

Chris - Yes indeed. Because when one watches, for example, and this is by no means a scientific <laugh> authority, Jurassic Park, your T-Rex type animal has a mouth full of teeth, all more of them outside than inside. Is that what you're getting at? The lips seem to have been forgotten in the mix.

Thomas - Yes, exactly. And Jurassic Park is itself kind of funny because at the time it came out they tried to do their best to make things very accurate and that was sort of the way people portrayed T-Rex at that time. But it's funny in that they made the T-Rex have no lips in that film, but all the other dinosaurs do, including things like the Velociraptors.

Chris - Yeah, I was thinking of precisely that. So why did this come about? Was this just because people wanted to create a fearsome image for dinosaurs and that fitted the bill or did they really have reason to suspect that because crocodiles have their teeth on show, that's probably what T-Rex did.

Thomas - I do think that making them sort of look superficially more fearsome must have played some kind of a role, even if it was unconscious. But on the scientific side, that question has been batted around a little bit over the years. And at the time they were making the film, yes, there had been some sort of brief discussion that, well, the closest living relative dinosaurs that actually have teeth are crocodilians. So that may be the most likely thing. And it wasn't interrogated much more than that. There was also the sort of related issue that some people had suggested things like T-Rex in particular, which had very, very large teeth, that the teeth simply might be too large to be covered by lips.

Chris - Why, when it comes to teeth, do lips matter?

Thomas - Well one of the things that lips do is not only does it protect the teeth from things banging into them, or help constrain their movement during feeding and whatnot, but it also keeps the teeth and the tooth enamel specifically, it keeps them protected and healthy because when they're hydrated they break down less quickly, the enamel. And so if it's exposed to air all the time, it becomes more brittle and it's more easy to crack and break.

Chris - And that's what you see in Crocodilians is it, if you look at your average big croc, their teeth are falling apart because they're exposed all the time?

Thomas - Yes and no. They definitely do break at a higher rate. But crocodilians are also interesting in that as an aquatic animal, as an animal that's specialized for living especially in the water while they're on land, their teeth will be exposed to air. But of course when they're in the water, they don't really have to worry about that quite as much.

Chris - So they get hydrated via a different mechanism. How did you try and probe what was going on with these ancient dinosaurs then? Because the difference is of course we can look at a crocodile today and get some insights, but we can't look at T-rex in the flesh.

Thomas - Right, exactly. And so we needed to use a mixture of sort of slightly more direct and then other indirect sources of evidence to try and get at testing those. One of those was a comparison, like I said, between looking at the teeth of crocodilians, in this case alligator, where we looked at how the enamel wears down on their teeth on the outside that faces the air outside of the mouth and the inside facing the inside of the mouth where they're actually still getting hydrated from their breath. And we saw that the enamel wears down very, very differently. On the outside it's way more worn down than the inside. And we did the same thing with the tyrannosaur and we saw that very unlike the alligator, the enamel is equal thickness on the inside and the outside. And that's much more consistent with what you would see in like a lizard or another animal that has lips covering its teeth when its mouth is closed.

Chris - So you actually went and got some fossil teeth remains of this dinosaur that had that detail preserved?

Thomas - That's right. Our primary samples was actually teeth that are called functional teeth, which means teeth that were found in the jaw of a specimen and were of sort of the right stage of development to have been actively being used by the animal at the time that it died, before it was fossilized. Another part that we looked at to get at that earlier sort of question of whether these teeth were simply too large to be fully covered by lips, we looked at some living animals today that also have very large teeth, and these are the monitor lizards. And so some of these actually have teeth that are proportional to the size of their skulls. They're larger than the teeth of something like a T-Rex, proportional to the size of its skull. And we looked at the way the teeth scale with skull size and all of these lizards, which we know for sure have lips. And then we saw that theropods, the sort of bipedal, often predatory dinosaurs like T-Rex, have a similar relationship between tooth and skull size as these lizards did, which also sort of provide some evidence that it was at least possible for them to have lips.

Chris - So your hypothesis then is that animals like T-Rex and its relatives would've had their teeth covered. So we're going to have to redraw these dinosaurs, presumably they're going to sort of break the mold, aren't they? People are going to have a job imagining them or at least recognizing them to start with?

Thomas - Yeah, I mean I think this evidence will certainly push things in that direction. There's been a number of different artistic reconstructions of things like T-Rex actually with lips in recent years. So I think it was sort of going that way a little bit already. But I think the evidence will certainly help push it more in that direction.