Theropod dinosaurs have often been given lizard-like labial glands covered with skin, colloquially called "lips". This is based on the presence of little foramina--holes in the skull that provide nutrients, paths for blood vessels and attachment for muscles and tendons. These little holes, the foramina, dot the edges of the lower part of the maxilla and premaxilla in both theropods and in lizards. The interesting thing, is that such foramina are also present in crocodiles and in birds, both of which do not have any type of flexible "lips". So here is a photo gallery of such animals:

First, the Lizards:


Iguana iguana


Dracaena guianensis


Varanus salvator


Varanus salvadorii


Cyclura cornuta

Next, the Theropods:


Daspletosaurus sp.


Velociraptor sp.


Tyrannosaurus rex - "Samson"


Tyrannosaurus sp. "Jane"



The birds:


Cathartes aura


Tachyeres brachypterus


Eudromia elegans


Procellaria aequinoctialis


Phalacrocorax sulcirostris


Rhyncheros jubatus


Chunga burmeisteri


Milvago chimachima


Tympanuchus phasianellus


Mergus merganser

Crocodylians:


Crocodylus niloticus


Alligator mississippiensis

Discussion:
Now that we've seen some skulls of various animals, it is interesting to note that birds and crocodylians both have small foramina close to the edge of the lower part of their maxillae. Yet neither of these groups have lips, but either have beaks or just tight, scaled skin.

So which is the better model for theropod dinosaurs? At this point, we can't be sure. But it is of note that the most closely related living taxa to dinosaurs (i.e., birds, crocs, etc.) don't have lips. Also, I'd say that the croc and bird mouth foramina look more similar in size, shape, and distribution to those of theropods than do the foramina of lizards. Albeit, the sample I have above is biased...I can't put every skull picture I find of each lizard, theropod, croc, and bird taxon. That said, I say the theropod mouth foramina distribution, size and number fall into the croc-bird range, not the lizard range.

Another interesting thing, is that the the presence of foramina lining the bones of the mouth does not appear to correlate with the presence or absence of lips. In fact, if you look at the skull of mammals, most of which have lips, you'll notice a conspicuous absence of foramina compared to lizards, dinosaurs, crocs and birds:



This chimpanzee skull, for instance, only has a few, large foramina. Most of which aren't in the area of the lips. Yet chimpanzees have very large lips, and are used quite expressively.

So what does this mean for theropods, and by extension, other dinosaurs? Well, to me it indicates that the preponderance of evidence indicates that they did not have lizard-like or mammal-like lips. To me, they probably had tight, possibly keratinous, skin around the edges of the mouth.

But what about the argument that lips would help protect the teeth from damage, rot, infection and dust and sand and other debris? Well, frankly, crocs get along fine without lips. Also remember, that theropods lost teeth on a regular basis and had new teeth behind each tooth ready to take the place should the current tooth fall out. In fact, theropod teeth are extremely common in the fossil record which would seem to indicate that they did lose them rather frequently--rather like sharks. The fact that many early birds had teeth, and did not have lips, would tend to also favor a lip-less hypothesis for theropods.

There is also another anatomical reason why some theropods probably could not have had lips: their teeth were simply too long!



The teeth in this Gorgosaurus go well past the line of foramina. This would mean the teeth would go past the lips, piercing right through them like the dinosaurian version of a babirusa!



Babyrousa sp.

In fact, the babirusa's teeth don't actually pierce through their lips, but [edit:]their skull the skin covering on the dorsal surface of their skull (the tooth socket is actually rotated, see Jaime Headden's comment below). But it's the closest thing I could think of to what a lip-piercing theropod would look like. Really, many theropods could not have had lips on their dentary, because the teeth would go past the nutrient foramina, and in some cases (like in Ceratosaurus), past the dentary, piercing the lips. If you notice in the lizard pictures above, lizard jaws don't close tightly as in theropods. Also, the top and bottom jaws rest nearly on top of one another, meaning the teeth in the upper jaw do no cover the foramina in the lower jaw. So, in my opinion, the jaw anatomy of theropods also contradicts a lips-hypothesis for theropods. Basically the lips-hypothesis boils down to only having lips on the cranium and not on the dentary. I know of no animal for which the lips are only limited to the head.

So basically, unless we find a theropod fossil mummy with the soft-tissues of the head exquisitely preserved, there is no reason to think theropods had lips. In fact, there is plenty of circumstantial evidence to indicate the opposite: theropods had no lizard lips! The extant-phylogenetic bracketing (EPB) method indicates no lips for theropods. Also, it does not appear that foramina are correlated with the presence of lips (see crocs and birds). Finally, lips are anatomically implausible for many theropods based on their jaw mechanics.

Finally, I apologize that I haven't referenced the scientific literature much on this. The fact is their are only two papers that deal with this subject that I know of, one by Lawrence Witmer and one by Tracy Ford. I may cover these papers next time in a follow-up post. Also, there is not much published on lizard lips that I could find using Google Scholar (and none that I had access to....scientific papers need to be open access!). Apparently, lips are not that interesting to most biologists and paleontologists. Oh well, I think it's an interesting topic, anyways!

Comments, as always, are welcomed!