Hadrosaur Jaws!

Hadrosaurs never really get the attention they deserve. Why? Well, probably because they don’t really have much in the way of cool spikes, armor, horns, or knife-like, serrated teeth as other dinosaurs do. They are many times referred to as the “Cows of the Cretaceous”, which, quite frankly, I disagree with entirely. Not that I don’t think cows are interesting in their own right, but hadrosaurs have so many other fascinating aspects of their biology that boggle the minds of paleontologists around the world. It’s hardly fair to compare them. Here, I am going to talk about a particular hadrosaur adaptation that has puzzled paleontologists most of all, for over a century: their crazy, tooth-infested feeding apparatus.

"Hadrosaurs" volume, edited by
David Evans and David Eberth.
Cover art by Julius Csotonyi.

A few months ago, I was lucky enough to have my paper entitled, “Hadrosauroid Jaw Mechanics and the Functional Significance of the Predentary Bone” published as a chapter in an Indiana University Press compilation volume by David Evans and David Eberth called "Hadrosaurs". In it, I discuss the anatomy of jaw elements in a few different hadrosaur genera to understand, qualitatively, how different elements were able to move against each other during feeding and what this meant for the entire jaw mechanism as a whole.

Hadrosaurs have a lot of teeth. I mean, a LOT of teeth. About 1,400 of them. They are stacked up on each other in up to around 40 columns and they continuously grow new ones throughout their lifetime. What’s even more interesting, though, is how they are oriented and associated with each other. The columns of teeth are pushed together from front to back (mesial to distal, for the jaw scientists out there) and, at the occlusal surface, they combine to form an elongate, flat surface on which the opposing jaw can bite. This huge platform occlusal surface gives room for it to move in different directions while chewing. Furthermore, the elongate combined occlusal surface of the lower teeth angle outward (i.e., toward the outside of the mouth) while the occlusal surface of the upper teeth are angled inward toward the tongue. This creates an angled occlusion, which would make it seem like the food would fall right out of the mouth.

A hypothesis of hadrosaur feeding mechanisms that was well accepted for over two decades, was proposed by my previous PhD mentor, Dave Weishampel (1984), as well as David Norman (1984) called “Pleurokinesis” (also see: Norman and Weishampel, 1985). Pleurokinesis is a feeding mechanism involving a domino effect of cranial elements causing the upper jaw, or maxilla, to rotate outward during occlusion while the lower jaw closed upward. This was proposed for various reasons, including mobility at certain joints within the skull as well as tooth wear that was oriented transversely, which was unusual for an animal with an occlusal surface that is angled outward. Recently, Pleurokinesis has been rejected by a few studies based on restrictions of cranial elements by other cranial elements, ligaments, and muscles (Holliday and Witmer, 2008; Rybczynski et al, 2008; Bell et al., 2009; Cuthbertson et al; 2012).

Previously proposed "Pleurokinesis" model
by Weishampel (1984); Norman (1984);
Norman and Weishampel (1985).
Figure from Nabavizadeh (2014).

Now, the dentary should be a more familiar bone for most anatomists and paleontologists. It is the largest bone in the lower jaw. A bone that might not be as familiar to many, however, is the predentary bone. The predentary bone is a single, midl).ne element that articulates with the front end of the dentary bones on either side, creating what is, functionally, the animal’s chin. With very few exceptions, predentaries are mainly only found in ornithischian dinosaurs, which include the horned ceratopsians, plated stegosaurs, armored ankylosaurs, dome-headed pachycephalosaurs, and billed ornithopods (including hadrosaurs) as well as their kin. Exactly what is this bone used for? Why did ornithischians evolve it and keep it for over 100 million years of evolution

The question still remains, though: how DID hadrosaurs “chew” with this tooth morphology? In my analysis of hadrosaur jaw bones, I tried to find out exactly how the bones articulated with one another and if there was potential movement between bones that maybe have not been emphasized before. The studies that rejected pleurokinesis briefly discussed potential mobility at a joint that was not thought of much before: the predentary-dentary joint, although there were no predentaries available in their analyses.

Hadrosaur predentary in different views.
Figure from Nabavizadeh (2014).

In this paper, I discuss the possibility that the predentary bone likely acted as an axial point at the midline as the two dentaries rotated around their long-axes simultaneously during “chewing”. One way they were likely capable of doing so is the lack of a clasping junction between the dentaries and predentary. Look at any museum specimen of a hadrosaur on display and you’ll see that it is really just hovering in front of the jaw with no clear connection with the rest of it. Funny, isn’t it? 

Proposed long-axis rotation of both sides of jaw.
Figure from Nabavizadeh (2014).

What does this mean? Well, it was probably rotating around slightly in a fibrocartilagenous joint capsule at the predentary-dentary junction. Remember, too, that it wouldn’t need to rotate around much at the front of the jaw to create a much larger rotation toward the back of the jaw with the angle they are associated in. Other aspects of the anatomy that suggest this rotation are the ball-to-cup articulation of the cranium with the jaw at the quadrate-articular jaw joint (how could it not be rotating??) as well as the tooth wear orientations showing multi-directional wear.

With this analysis, I suggest a feeding mechanism that starts with a front to back palinal chewing motion followed by a “bolt-cutter-like” cutting of vegetation rotating into the mouth on both sides of the jaw simultaneously. This type of feeding mechanism would have been difficult without the aid of another bone such as the predentary to keep the jaws together. Just another example of how wonderfully bizarre hadrosaurs really are.

Proposed full feeding mechanism (Nabavizadeh, 2014)

All of this isn't to say that there is a lot more involved in hadrosaur jaw mechanics than just how their bones are put together. There are so many other important factors in hadrosaur feeding, not the least of which is their crazy jaw musculature. This study specifically opens up a lot of questions as to the function of the predentary bone in ornithischian dinosaurs as a whole and why it’s so important. My dissertation expanded upon this and other scenarios in ornithischian feeding, so more papers will be coming your way soon! Stay tuned.

References:

Bell, P. B., E. Snively, and L. Shychoski. 2009. A comparison of the jaw mechanics in

hadrosaurid and ceratopsid dinosaurs using finite element analysis. Anatomical Record 292:1338–1351.

Cuthbertson, R. S., A. Tirabasso, N. Rybczynski, and R. B. Holmes. 2012. Kinetic

limitations of intracranial joints in Brachylophosaurus canadensis and Edmontosaurus regalis (Dinosauria: Hadrosauridae), and their implications for the chewing mechanics of hadrosaurids. The Anatomical Record 295:968-979.

Holliday, C. M., and L. M. Witmer. 2008. Cranial kinesis in dinosaurs: intracranial joints,

protractor muscles, and their significance for cranial evolution and function in

diapsids. Journal of Vertebrate Paleontology 28:1073–1088.

Nabavizadeh, A. 2014. Hadrosauroid jaw mechanics and the functional significance

of the predentary bone. In: The Hadrosaurs: Proceedings of the International Hadrosaur Symposium (D. Evans and D. Eberth, eds), Indiana University Press, Bloomington.

Norman, D. B. 1984. On the cranial morphology and evolution of ornithopod dinosaurs.

Symposium of the Zoological Society in London 52:521–547.

Norman, D. B., and D. B. Weishampel. 1985. Ornithopod feeding mechanisms: their

bearing on the evolution of herbivory. American Naturalist 126:151–164.

Rybczynski, N., A. Tirabasso, P. Bloskie, R. Cuthbertson, and C. Holliday. 2008. A

three-dimensional animation model of Edmontosaurus (Hadrosauridae) for testing chewing hypotheses. Palaeontologica Electronica 11(9A).

Weishampel, D. B. 1984a. Evolution of jaw mechanisms in ornithopod dinosaurs.

Advances in Anatomy Embryology and Cell Biology 87:1–109.