If you have ever seen an artistic depiction of a Mosasaur, you would know that they had very large teeth. And why shouldn’t they? After all, they are prehistoric predators, and if a predator doesn’t have teeth showing, there is definitely a drop in the “cool factor” of the animal. This was the reason behind the shrink wrapping of prehistoric animal mouths that until very recently left dinosaurs with exposed teeth when their mouths were closed. This changed due to an exceptional study by Robert Reisz revealed that the teeth were hidden by fleshy gums when their mouths were closed(1).
But what if the same type of affliction that affected dinosaur illustrations also affected Mosasaurs? And I’m not just talking about the presence of lips, because there have been some really exceptional illustrations showing Mosasaurs with fleshy lips by artists such as Dan Varner, Julius Csotonyi and Andrey Atuchin (if there are other reconstructions you think good, let me know in the comments). What I am talking about is the absolute lack of visible teeth.
Let’s look at where Mosasaurs sit phylogenetically. They were without a doubt Squamates, with most recent analyses placing them as a sister group to snakes. However, their direct placement does not matter, as all analyses place them within the group Toxicofera, which includes snakes, Monitor Lizards, Gila Monsters, Iguanians, the extinct Polyglyphanodonts and others(2) (many of these are now considered venomous, but due to jaw capabilities and lack of venom delivering system that aquatic venomous snakes have, Mosasaurs were likely not). An interesting feature of the extant members of this group that are carnivorous is that their teeth do not show when their mouths are open, even the carnivores that are aquatic or semi aquatic(3)(4)(5).
Let us look at two examples, one Anguimorph and one snake. The Asian Water Monitor, Varanus salvator, is semi aquatic, with a diet that includes a variety of both terrestrial and aquatic prey. It is even possible for them to remain submerged underwater for up to half an hour when hunting(6). And yet, when they open their mouths, their teeth are completely invisible, hidden by a large layer of gums(3). This is evident in images here and here.
Next, let us look at a modern aquatic snake. While sea snakes would be excellent models, unfortunately their oral anatomy is not well documented. An alternative model is supplied in the form of the Water Moccasin or Cottonmouth, Agkistrodon piscivorus. This mostly aquatic snake is mainly freshwater and has a diet ranging from fish to amphibians, and even other reptiles, such as crocodilians or other snakes, sometimes even other Water Moccasins(7) (this behaviour is mirrored in Mosasaurs(8)). Their teeth too are covered in gums(4), as is evident here, here, and in virtually every image of a Water Moccasin with an open mouth.
These two individuals are just the best-documented individuals I could find of their respective groups. As most species of these groups, as well as those that lived terrestrially, had tooth-covering gums, chances are it is a trait that was present in all of the Toxicofera, and therefore the Mosasauria, at least at the earliest stages of their evolution. And since these aquatic members retained them, there seems to be no motive for them to be lost in Mosasaurs.
Of course, Mosasaur skulls are very different in many respects from Anguimorphs and snakes. It can be argued that the teeth of Mosasaurs are much too large to be encased in gums, so let’s examine the skull morphology.
Here the skull of the Komodo dragon, Varanus komodoensis, is compared to that of Tylosaurus sp. The size of teeth relative to the skull is either equal in both skulls or larger in that of V. komodoensis, and yet the teeth of V. komodoensis are completely covered by gums(5), as is seen here. This also occurs with Water Monitors, although this feature is difficult to measure in snakes due to the morphological and structural differences needed for the swallowing of larger prey changing the skulls substantially from the Mosasaur-Anguimorph model(9). The lower jaws of aquatic snakes do maintain this resemblance, and can be seen for the Water Moccasin(10) here (top skull).
Phylogeny shows that Mosasaurs should have retained tooth-covering gums in at least the earliest stages of Mosasaur evolution, and the presence of them in modern aquatic Toxicoferans and the morphology of the skull shows that there is no reason to suggest they lost this feature. This means that unless there was some pressure pulling the gums back, the teeth of Mosasaurs would likely have been hidden by gums when their mouths were open.
Tell me in the comments what you think, and if you have any additions or subtractions for this theory, let me know. I’d be glad to incorporate more information to this idea to gain a better understanding of these incredible creatures.
That’s all for now. See you soon,
1 Eligh, Blake. “Did Dinosaurs Have Lips? Ask This University of Toronto Paleontologist.” University of Toronto. N.p., 19 May 2016. Web. 02 Oct. 2016.
2 Reeder, Tod W., Ted M. Townsend, Daniel G. Mulcahy, Brice P. Noonan, Perry L. Wood, Jack W. Sites, and John J. Wiens. “Integrated Analyses Resolve Conflicts over Squamate Reptile Phylogeny and Reveal Unexpected Placements for Fossil Taxa.” PLoS ONE 10.3 (2015): n. pag. Web. 2 Oct. 2016.
3 Kucherova, Anna. Asian Water Monitor with Open Mouth. Digital image. Dreamstime. N.p., n.d. Web. 2 Oct. 2016.
4 Conrad, Jim. Digital image. Backyard Nature. N.p., 29 Oct. 2009. Web. 2 Oct. 2016.
5 Digital image. Animal Unique. N.p., 10 Apr. 2012. Web. 2 Oct. 2016.
6 Byers, Doug. “Varanus Salvator (Common Water Monitor).” Animal Diversity Web. N.p., n.d. Web. 02 Oct. 2016.
7 Szalay, Jessie. “Facts About Water Moccasin (Cottonmouth) Snakes.” Live Science. N.p., 21 Nov. 2014. Web. 2 Oct. 2016.
8 Pappas, Stephanie. “Full Belly Fossil! ‘Sea Monster’ Had 3 Others in Its Gut.”Live Science. N.p., 1 Nov. 2013. Web. 2 Oct. 2016.
9 Lee, M. S. Y., Bell,Gorden L., Jr, & Caldwell, M. W. (1999). The origin of snake feeding. Nature, 400(6745), 655-659. doi:http://dx.doi.org/10.1038/23236
10 Cottonmouth Skull (top) vs. Eastern Ratsnake Snake Skull (bottom). Digital image. Virginia Herpetological Society. N.p., n.d. Web. 2 Oct. 2016.