Tuesday, June 30, 2009

Nom nom nom

Above: Edmontosaurus skull, note teeth in back of jaw. Photo: Ballista / Wikipedia, licensed

Did hadrosaurs chew? A recently named lambeosaur called Angulomastacator ("angle chewer") would have us think so. So would a new paper by Williams et al. on an intensive study of hadrosaur tooth wear. As this article reports, the direction of hundreds of scratches were mapped in 3D to suss out what type of chewing method was used by hadrosaurs. The authors found that they employed an extinct form of chewing unlike any living animals, in which the natural kinesis (joints between the skull bones found in modern reptiles) forced the bones of the upper jaw outward, causing the tooth batteries to slide against each other in a sideways motion.

Sounds very interesting and a great step to resolving the ecologies of this well-known dinosaur group... or is it? A major paper that came out in the December issue of JVP argued convincingly that dinosaur skulls such as this didn't actually have any kinesis at all! As David Marjanovic summed up on the DML, the argument there was that people who have spent their careers studying modern reptiles and their kinetic skulls have assumed that dinosaurs followed the same pattern, and have grasped at straws to find evidence of kinesis where it doesn't actually exist (that is, that dinosaurs lost cranial kinesis at some point in their evolution from other reptiles, or never had it, meaning it's a novel feature of lizards and snakes). If that's the case, than the underlying assumption of this new hadrosaur paper is wrong. Another blow against their conclusions, as noted in the article above, is that reports of fossil stomach contents from hadrosaur mummies show food that had been sheared, not chewed. Gut contents also suggest a diet predominantly of conifers (pine trees etc.), while the tooth wear study suggests food was cropped close to the ground and may have been rich in silica, suggesting a diet primarily of ferns and horsetails.

Above: Edmontosaurus tooth battery, which was studied for wear patterns. Photo: Vince Williams / Univ. of Leicester

So then, how to explain the unique wear patterns on hadrosaur teeth? How can we rectify the conflicting evidence regarding diet? Surely another salvo in this controversy will appear in print in a few months.


  1. For a better understanding of cranial kinesis in reptiles, I'd recommend the following reference:

    Metzger, K. 2002. Cranial kinesis in lepidosaurs: skulls in motions. In "Topics in Functional and Ecological Vertebrate Morphology", pgs. 15-46, Aerts et al. (eds.), Shaker Publishing.

    For that "major paper that came out in the December issue of JVP", see:

    Holliday, C. M. and Witmer, L. M. 2008. Cranial Kinesis in Dinosaurs: Intracranial Joints, Protractor Muscles, and Their Significance for Cranial Evolution and Function in Diapsids. Journal of Vertebrate Paleontology, 28 (4): 1073-1088.

    If you do not have a personal subscription (through SVP) or an institutional subscription, I'd be happy to email a copy of this to anyone who wants it.

    Given that the Williams et al. paper had only just entered into review in December 2008, it is disappointing that it did not discuss the Holliday and Witmer paper, but perhaps understandable. However, Rybczynski et al. (2008) also objected to the pleurokinetic model, if you have read their paper. With two excellent papers coming out in 2008 discussing the problems of the pleurokinetic model of mastication in ornithopods, I am left wondering how such an oversight could occur. :-)

  2. I don't have any of the relevant material in front of me, so can anyone shed any light on how the end results of grinding by making the lower jaw do the slipping and sliding (movement at the quadrates and predentary), insead of the maxillae, would look on the teeth and food?

    I found the indication that it was eating leaves and not twigs and such very interesting (of course, I've got a personal interest in that point ;) ).

  3. Given the time we would have discussed Holliday & Witmer 2008 and we did discuss Rybczynski et al. 2008, both excellent papers. No oversight however, as neither would have changed the pattern of scratches that we found on the teeth. Our work isn’t based on any assumptions regarding skull function, but on the actual record of tooth movements during life.