Monday, January 6, 2014

Stegosaurs, Ankylosaurs, and Kin: A Look at the Crazy, Crazy Jaws of Armored Dinosaurs

Hello again!

I've finally decided to post some more illustrations I'm doing for my dissertation...

These are illustrations I've done for my chapter on the jaws of Thyreophora (stegosaurs, ankylosaurs, and kin). It's one of my descriptive chapters which I've spent a lot of time trying to figure out what should go where and what is important to illustrate that would need more than just words alone.

Thyreophorans, or the "armored dinosaurs", are known to be some of the most bizarre dinosaurs to have ever existed. Why? Because not only do they have bones on the inside, but they've got bones on the outside, too. It is what is known as dermal bone, meaning bone that develops in the skin. Now, there are a lot of vertebrates living today with dermal bone which serve the purpose of (among others) a shielding protection against predators (i.e., turtles, armadillos, and loads more). Thyreophoran dinosaurs take this to the extreme and are covered with plates and scutes, some to the point of making them look like enormous tanks that could kick any predator's--well, you know.

This post will just show illustrations I've made for the chapter. In a future post, I will talk more about how these critters used their jaws:


Basal Thyreophorans

So the most basal (earliest forms) of thyreophorans consisted of things like Lesothosaurus (although, maybe not anymore?), Scelidosaurus, Scutellosaurus, and Emausaurus. I've only done illustrations for Lesothosaurus and Scelidosaurus, though, because that was all I was really able to observe for this study.


This is Lesothosaurus, one of the first thyreophorans to have existed. It's a tiny little guy that walked on two legs and a pretty long, skinny snout. I've decided to do side views of skulls in micron pen to show, with line-work, the different morphologies on the skull.


 Lesothosaurus had your standard leaf-shaped teeth with little, tiny sharp projections coming out the top edge of them (on left below). These teeth are ideal for cropping plant material, especially for such a small dinosaurs going after tougher plants. They also had teeth at the front of the upper jaw (premaxillary teeth; on right below) that were good for puncturing plants when first nipping them, like our incisors. I've done these and many other illustrations of different elements in graphite pencil which I then put into Photoshop and played with contrast and textures.



There is a bone at the front of the jaw (which you've all probably heard me boast about at some point or another) which is unique to all of the dinosaurs I am studying (within the group Ornithischia, which Thyreophora is a part of). It is a little bone at the middle of the front of end of the lower jaw that might have been used for mobility at this junction. I'll get to that in another post, though. For now here are some views of the predentary in Lesothosaurus (overhead view on left; side view on right):

 And here's an underside view looking at the predentary-dentary bone articulations (based on Sereno, 1991):


I've also included a view drawings of Scelidosaurus material as well. Scelidosaurus is a little more derived than Lesothosaurus and has a lot more dermal bone, or armor, that surrounded its body. On the left below is the jaw joint articulation (quadrate bone of skull nested within jaw joint). On the right is a side view of the back of its lower jaw, showing the coronoid eminence and retroarticular process morphologies.



Aaaaaaand just for kicks, here is a Scelidosaurus tooth! More spade shaped and bulbous.

Stegosaurs

The next group I talk about is stegosaurs. Everyone knows stegosaurs, right? It means "roof lizard" and is named so because of its insane-looking huge plates sticking out of its back (and huge spikes on its tail). These were enormous creatures that walked on all fours and had incredibly tiny heads for such a big body. Below is an ink illustration of a Stegosaurus skull:


The teeth in stegosaurs are also more-or-less leaf shaped. They have ridges on the sides and little more subtle bumps along the top edge (seen below on left). Premaxillary teeth (shown below on the right) are only seen in Huayangosaurus, the basal-most known stegosaur. More derived stegosaurs, however, have evolutionarily lost premaxillary teeth.


The predentary of Stegosaurus is also shown below (side view on top left; overhead view on bottom left) and the predentary-dentary articulation at the front of the jaw (view from below on right):



I've also depicted the jaw joint from behind. You can see that it is bicondylar (two spheres) like in Scelidosaurus above and rests in a cupped surface in the mandible.

Ankylosaurs

Lastly, the beloved ankylosaurs, or "fused lizard". (By the way, don't take anything away from the term "lizard" when I say it in a name meaning. Dinosaurs aren't lizards. It's just etymology. Ha!)

Ankylosaurs are the huge tanks of armor. They have so much plating and scutes on their backs that no predator could have done anything to it from above. All it would have to do is hunker down and and Ankylosaurus could be chomping on a little bush as Tyrannosaurus kept trying to peck at it (given it was stupid enough to do so, which I don't think so... Maybe?) Some (not all) of them also had club tails that they used for EXTRA protection. Below is an ink drawing of the skull of Euoplocephalus (without the predentary):


Okay... I just have to point out that these guys have THE most bizarre jaws I have ever seen. Their tooth rows are insanely curved. But again, I'll get to that in a later post. For now, here are images of the teeth. On the left is an example of one of the dentary teeth that run along the jaw line. They look very similar to stegosaur teeth, except their little projections on the top, or denticles, are a lot sharper and more specialized for slicing and dicing. On the right is an example of a premaxillary tooth, which is only seen in some of the basal-most ankylosaurs, but not most of them (both teeth are illustrated from Silvisaurus):

 

The predentary bone in ankylosaurs is really weird too, because really all it is is a single bar that runs along the front of the jaw and hardly connects well with the rest of the jaw at all.

 

(Overhead view of Euoplocephalus predentary)


 

(View from behind Euoplocephalus predentary showing flat articular surface that would press up against the two dentaries)



(View from beneath front of Pinacosaurus jaw showing predentary articulation with dentaries [note: this predentary is more arched than in Euoplocephalus)


Finally, below I've illustrated the jaw joint of ankylosaurs (in this case Edmontonia) from behind. The quadrate (of the skull) is very wide at the bottom and rests in a very open basined jaw joint surface of the mandible which is also very curved.


Anyway, there it is! Again, in another post in the future I'll talk more about the jaw mechanisms these guys were using, but for now I just wanted to get these illustrations out there. Feedback would be great! 

Thanks for stopping by!

~Ali

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