We’ve all seen artwork of the mighty Tyrannosaurus  rex, roaring and showing off its massive teeth.

But it turns out that researchers are a bit more   interested in what those teeth looked  like when their mouths were closed. Specifically, if it had lips  covering those chompers.

Believe it or not, the question of whether T. rex   had lips has been the subject of  many years of scientific debate. And if we can settle this argument, we can  answer an even more important question,   one we’ve definitely all wondered: if I  give a T. rex a kiss, could it kiss me back? [♪ INTRO] Lips are kind of important. If you don’t believe me, try going a whole  day with your lips pulled back - your mouth   will dry out, you’ll drool all over yourself,  and no one will want to eat a meal with you.

You wouldn’t even be able to give kisses! Aside from smooching, animals use their lips for  all sorts of functions - holding food and water,   communicating with each other, even  manipulating things in their environment. But obviously, not all animals have lips.

Birds have their beaks and crocodiles  have those long, lipless snouts,   so you can see their pointy teeth  even when their mouths are closed. So, how do we tell where fossil animals like  the T. rex fall on the lips vs no lips divide? Like with all other debates, we start  with their closest living relatives,   which in this case, are birds and crocs.

We know from fossils that T. rex didn’t  have a bird beak, but it is possible that   they had mouths like crocs, with no  lips and permanently exposed teeth. On the other hand, those crocodiles  might not be the best comparison here,   since they live a very particular lifestyle,  They spend most of their time in the water,   and they use their snouts to feel around for food. In contrast, dinosaurs like T. rex primarily  lived on land, so you can imagine that if   they’re doing different things with their faces,  they might need different soft tissue to do that.

So instead, we can look at some more distant  relatives and talk about reptiles in general. Most of our modern land-dwelling reptiles are  lizards and snakes, and they do have lips. So even though they’re not as closely related,  maybe their similar lifestyles make lizards a   better example of what to expect of the  kissing capabilities of tyrannosaurs.

So, here we have our two main options: T. rex was  most likely either croc-faced or lizard-faced. And paleontologists have been flapping  their lips on the subject for a long time. One of the classic arguments against  lips is the sheer size of T. rex teeth,   since they can be up to a foot long.

And the argument was that this is simply far  too large to be contained inside of lips,   so at the very least, the mega-sized  teeth must’ve been out there all the time. But this argument hasn’t held  up to scientific scrutiny. T. rex teeth might be huge,  but so are their heads.

Close examination has revealed that  the relative size and position of T.   rex teeth are pretty similar to big  monitor lizards like Komodo dragons,   and their teeth aren’t too big  to be covered up by scaly lips. So we can infer that a T. rex would at least  be capable of having big ol’ lips, too. Meanwhile, one of the biggest points  on the pro-lips side of the debate is   that those big teeth would need to be protected.

T. rex teeth, just like yours  and mine, are covered in enamel. Enamel is a very durable tissue - it’s what makes  our teeth so good at crushing other materials. But if the enamel dries out, it can  become brittle and easily damaged.

One of the major benefits of  having lips is that they seal   in moisture and keep our teeth damp and strong. Crocodiles don’t have lips, but  because they live in the water,   their exposed teeth still  get plenty of moisturizing. Being a land animal, T. rex’s mouth was  constantly exposed to the air and sunlight.

Their teeth would have been in  danger of drying out and cracking,   so surely they must have had lips  to keep their teeth moistened. But some scientists have pointed out  another possible dental workaround. One thing that T. rex and crocodiles do have  in common is rapidly replacing their chompers.

They both have, or had,   a constant new supply of teeth waiting in  the wings when the current tooth fell out. So maybe exposed teeth weren’t  that big a deal for T. rex,   because any damaged teeth could  be quickly replaced by fresh ones. As you can see, there isn’t a  clear-cut answer to this question.

In trying to resolve this debate, scientists  have looked at dinosaur skulls, jaws,   and teeth from every angle, comparing  and contrasting with living species,   but this research has given conflicting results,  leaving us without a definitive solution. Fortunately, the end of this  debate might not be too far off. But before we get into that,  all research needs funding,   even when it’s about kissing  dinosaurs.

So here’s a quick ad break. Thanks to Brilliant for  supporting this SciShow video. Brilliant is an interactive online learning   platform that lets you make  progress at your own pace.

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To try Brilliant for free,  visit Brilliant.org/SciShow,   scan the QR code on screen, or click  on the link in the video description. You also get 20% off an  annual premium subscription. Looking closely at fossil T. rex  skulls, researchers are able to   tease out a few key clues that may  solve this debate once and for all.

For starters, if you look really closely  at the area right above the T. rex’s teeth,   you can see these tiny pinprick holes. When the dinosaurs were alive, these tiny spaces   were the passages for various nerves and  blood vessels for their jaws and snouts. These nerves can both provide important  sensory feedback for the gums, skin, and lips,   and tell the muscles of the face how to move,  which is critical for any potential kissing.

We have these holes for the nerves  too, and so do modern reptiles. It’s part of what makes our  faces so sensitive and flexible. Comparing between living and extinct  species, scientists have found that the   tiny face-holes of T. rex are similar  to land-dwelling, lip-having lizards.

For example, tyrannosaurs feature a large  passage for the glossopharyngeal nerve,   which is associated with saliva production,   and that’s important for moistening  the teeth behind the lips. And this is also really different  from what crocodiles are rocking. Crocs have a very extensive array of nerves that  make the skin on their faces super-sensitive.

This gives them the superpower of being able  to detect small movements and tiny changes in   water pressure, which is particularly handy  for finding food in murky aquatic settings. We see a similar condition in the  skulls of some ancient aquatic reptiles,   and even in some modern birds  that hunt for food in the water,   but we don’t tend to see it among land-dwelling  species… even in the crocs’ own family tree. The early ancestors of crocodilians lived on land,   and lots of them left behind  skulls in the fossil record.

From these remains, paleontologists have found  that early crocs had a simpler series of facial   canals, less like their modern cousins  and more like lizards and dinosaurs. Even the teeth of those early crocs  were similar to carnivorous dinosaurs. And this drives home a really important point,   which is that modern-day  crocodile skulls are kinda weird.

Over the course of their evolution,  crocodiles and alligators have become   highly specialized for lives  of aquatic ambush hunting. As a result, their faces are  unusual compared to other reptiles. And that means that the lipless mouth of a  croc probably isn’t the best representation   for what to expect from T. rex, which  probably had a more typical reptilian face.

But if we want to know whether or not  tyrannosaur teeth were protected by lips,   it stands to reason that the best  place to look is the teeth themselves. If T. rex teeth were regularly exposed to  the elements, we might expect to see special   adaptations to keep them safe, and we might  also see evidence of damage on their teeth. In modern alligators, for example,  the enamel coating on their teeth is   thicker toward the tips, which are the  parts that are always out in the open.

And their teeth tend to develop highly worn outer  surfaces from all that damage due to exposure. In other animals, like lizards and us,  the teeth have only a thin coat of enamel,   which doesn’t end up wearing down in the same way. And then there are the extra-large exposed teeth,   like the big tusks of boars and elephants,  which have lost their enamel altogether.

I guess that’s one way to solve the problem. Well, when researchers took a close look at T.  rex teeth in a 2023 study, they found a thin layer   of enamel, and no major differences in damage  on the inner and outer surfaces of the teeth. That is to say, T. rex teeth are most  similar to those of animals that have lips.

All in all, the evidence seems to be building  in support of T. rex having lizard-like lips. And that means we have an answer  to our pressing question: Yes,   you probably could give a tyrannosaur  a smooch right on the lips! But what we haven’t addressed is whether  they could pucker up and kiss you back.

See, even though our lips are super mobile,   it’s pretty unlikely that a T. rex would  have had our level of facial flexibility. For one thing, reptiles don’t have  the sort of facial musculature that   we do that makes our flexible lips,  capable of smiling and puckering. T. rex probably had more stationary  lips like we see in lizards.

But oddly enough, they still seemed to  do a bit of romantic mouth-to-mouth,   just not in the way you’d expect. See, tyrannosaurs seem to have been in the  habit of biting each other on the face. In addition to all of the normal holes  and ridges in their skulls, T. rex snouts   are commonly covered in scars that match the  size and shape of the teeth of other T. rex.

These marks show signs of re-grown bone,  meaning the animals weren’t killed by   these bites, and they continued  on living and were able to heal. Facial scars are super common on T. rex skulls,   but they’re only seen in adult  individuals and never in young ones,   which some researchers suggest means face-biting  is a behavior related to sexual maturity. So maybe these were tender T. rex love bites?

A bit aggressive for my  tastes, but to each their own. Another more likely explanation is that this  is how they fought each other over territory or   mating rights… I mean, when your arms are that  tiny, you have to make do with what you have,   and what T. rex has is a giant  mouth full of banana-sized teeth. We see a similar territorial behavior,  and a similar pattern of facial scarring,   in modern and fossil crocodilians.

So even though T. rex and crocodiles  might be different when it comes to lips,   they’re not so different when it comes to  chomping on the faces of their neighbors. The lips debate isn’t over, and  there’s surely more to discover   about the facial features  of these ancient predators. For now, though, it looks like there’s a good  chance that T. rex sported a set of scaly lips.

And that means that yes, you probably  could kiss a T. rex on the lips. But until we know more about their  behavior… maybe it’s safer not to. [♪ OUTRO]