The Ornithischian Revolution

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The Ornithischian Revolution

Thursday, May 7, 2015 Hadrosaur Jaws!Hadrosaurs never really get the attention they deserve. Why?Well, probably because they dont really have much in the way of cool spikes,armor, horns, or knife-like, serrated teeth as other dinosaurs do. They aremany times referred to as the Cows of the Cretaceous, which, quite frankly, Idisagree with entirely. Not that I dont think cows are interesting in theirown right, but hadrosaurs have so many other fascinating aspects of theirbiology that boggle the minds of paleontologists around the world. Its hardlyfair to compare them. Here, I am going to talk about a particular hadrosauradaptation 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, HadrosauroidJaw Mechanics and the Functional Significance of the Predentary Bone publishedas a chapter in an Indiana University Press compilation volume by David Evansand David Eberth called "Hadrosaurs". In it, I discuss the anatomy of jaw elements in a fewdifferent hadrosaur genera to understand, qualitatively, how different elementswere able to move against each other during feeding and what this meant for theentire 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 theycontinuously grow new ones throughout their lifetime. Whats even moreinteresting, 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 toform an elongate, flat surface on which the opposing jaw can bite. This hugeplatform occlusal surface gives room for it to move in different directionswhile chewing. Furthermore, the elongate combined occlusal surface of the lowerteeth angle outward (i.e., toward the outside of the mouth) while the occlusalsurface of the upper teeth are angled inward toward the tongue. This creates anangled occlusion, which would make it seem like the food would fall right outof the mouth.

A hypothesis of hadrosaur feeding mechanisms that was well accepted for overtwo decades, was proposed by my previous PhD mentor, Dave Weishampel (1984), aswell as David Norman (1984) called Pleurokinesis (also see: Norman andWeishampel, 1985). Pleurokinesis is a feeding mechanism involving a dominoeffect of cranial elements causing the upper jaw, or maxilla, to rotate outwardduring occlusion while the lower jaw closed upward. This was proposed forvarious reasons, including mobility at certain joints within the skull as wellas tooth wear that was oriented transversely, which was unusual for an animalwith an occlusal surface that is angled outward. Recently, Pleurokinesis hasbeen rejected by a few studies based on restrictions of cranial elements byother 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 andpaleontologists. It is the largest bone in the lower jaw. A bone that might notbe as familiar to many, however, is the predentary bone. The predentary bone isa single, midl).ne element that articulates with the front end of the dentarybones on either side, creating what is, functionally, the animals chin. Withvery few exceptions, predentaries are mainly only found in ornithischiandinosaurs, which include the horned ceratopsians, plated stegosaurs, armored ankylosaurs,dome-headed pachycephalosaurs, and billed ornithopods (including hadrosaurs) aswell as their kin. Exactly what is this bone used for? Why did ornithischiansevolve it and keep it for over 100 million years of evolution
The question still remains, though: how DID hadrosaurs chew with this toothmorphology? In my analysis of hadrosaur jaw bones, I tried to find out exactlyhow the bones articulated with one another and if there was potential movementbetween bones that maybe have not been emphasized before. The studies thatrejected pleurokinesis briefly discussed potential mobility at a joint that wasnot thought of much before: the predentary-dentary joint, although there wereno 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 actedas an axial point at the midline as the two dentaries rotated around theirlong-axes simultaneously during chewing. One way they were likely capable ofdoing so is the lack of a clasping junction between the dentaries andpredentary. Look at any museum specimen of a hadrosaur on display and youllsee that it is really just hovering in front of the jaw with no clearconnection with the rest of it. Funny, isnt it?

Proposed long-axis rotation of both sides of jaw.
Figure from Nabavizadeh (2014).What does this mean? Well, it was probably rotating aroundslightly in a fibrocartilagenous joint capsule at the predentary-dentaryjunction. Remember, too, that it wouldnt need to rotate around much at thefront of the jaw to create a much larger rotation toward the back of the jawwith the angle they are associated in. Other aspects of the anatomy thatsuggest this rotation are the ball-to-cup articulation of the cranium with thejaw at the quadrate-articular jaw joint (how could it not be rotating??) aswell 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 predentarybone in ornithischian dinosaurs as a whole and why its so important. Mydissertation 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 hadrosauridand ceratopsid dinosaurs using finite element analysis. Anatomical Record 292:13381351.
Cuthbertson, R. S., A. Tirabasso, N. Rybczynski, and R. B.Holmes. 2012. Kinetic limitations of intracranial joints in Brachylophosauruscanadensis and Edmontosaurus regalis (Dinosauria: Hadrosauridae),and their implications for the chewing mechanics of hadrosaurids. TheAnatomical Record 295:968-979.
Holliday, C. M., and L. M. Witmer. 2008.Cranial kinesis in dinosaurs: intracranial joints, protractor muscles, andtheir significance for cranial evolution and function in diapsids. Journal ofVertebrate Paleontology 28:10731088.
Nabavizadeh,A. 2014. Hadrosauroid jawmechanics and the functional significance of thepredentary 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 cranialmorphology and evolution of ornithopod dinosaurs. Symposium of theZoological Society in London 52:521547.
Norman, D. B., and D.B. Weishampel. 1985. Ornithopod feeding mechanisms: their bearing on the evolutionof herbivory. American Naturalist 126:151164.
Rybczynski,N., A. Tirabasso, P. Bloskie, R. Cuthbertson, and C. Holliday. 2008. Athree-dimensionalanimation model of Edmontosaurus(Hadrosauridae) for testing chewing hypotheses. Palaeontologica Electronica11(9A).
Weishampel, D. B. 1984a. Evolution ofjaw mechanisms in ornithopod dinosaurs. Advances in AnatomyEmbryology and Cell Biology 87:1109.

Posted byAli Nabavizadehat8:33 AMNo comments: Email ThisBlogThis!Share to TwitterShare to FacebookShare to Pinterest Older PostsHomeSubscribe to:Posts (Atom)Blog Archive 2015(1) May(1)Hadrosaur Jaws! 2014(4) November(1) March(2) January(1) 2013(6) December(2) November(1) August(1) July(2)About MeAli NabavizadehI am currently a postdoctoral scholar in the Department of Organismal Biology and Anatomy and the University of Chicago. Previously, I received my PhD at the Johns Hopkins University School of Medicine in the Functional Anatomy and Evolution department. My interests include dinosaur paleobiology and vertebrate paleontology/comparative anatomy as a whole, specifically the evolution of jaw mechanics in ornithischian dinosaurs and craniofacial musculature of vertebrates.View my complete profile

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