Jawing about Dinosaurs: What the Herbivore’s Jaws Are Telling Us

One of the great things about working at a museum is having access to the vast collections of fossils that reside behind the scenes. These aren’t regularly seen by the public (our annual Open House excluded), but they nevertheless serve an important role in my research. I recently published just such a study in the Journal of Vertebrate Paleontology with my former Ph.D. supervisor, Dr. Jason Anderson, that draws heavily upon specimens in the collections of the Canadian Museum of Nature and elsewhere.

Panoramic view of shelving ranges containing dinosaur fossils.

A view of our vast fossil collections. Image: Jordan Mallon © Canadian Museum of Nature

We were curious to know what role, if any, differences in jaw mechanics may have played to allow plant-eating dinosaurs to co-exist in Alberta 75 million years ago.

We know today, for example, that the high diversity of finch species in the Galápagos Islands is made possible by their diverse jaw arrangements. Species that eat the largest, hardest seeds have larger jaw muscles that are situated further forward on the beak, giving those species the extra leverage needed to crack open such massive seeds. In this way, an ecosystem can support a rich diversity of species because they are not all competing for the same types of food.

We figured that maybe similar principles allowed for the rich diversity of plant-eating dinosaurs in Alberta during the Late Cretaceous Period.

It can be difficult to know about muscle size in the fossil record because these soft tissues do not normally fossilize. We can, however, know something about how the jaw muscles were arranged because they leave distinctive marks on the skull bones where they attach. Therefore, by carefully measuring the attachment sites of the different jaw muscles, we can come up with complex lever models of the lower jaw that allow us to compare bite leverage across numerous species.

Three diagrams of dinosaur skulls in lateral view; one diagram in lateral view of a lower jaw.

Jaw muscle reconstructions for an ankylosaur (A), a ceratopsid (B) and a hadrosaur (C). The lever model we used is depicted at right (D). Image: © Journal of Vertebrate Paleontology

What did we find? It seems that the jaws of the duck-billed hadrosaurs and horned ceratopsids were constructed in such a way as to produce a very high leverage, giving them an especially powerful bite. The armoured ankylosaurs, on the other hand, had comparatively weak jaws.

Within each of these groups, bite forces do not appear to have varied by much; different species of hadrosaurs had jaws that worked in very similar ways, as did different species of ceratopsids. The jaws of different ankylosaur species were slightly more variable, suggesting that there may have been some variability in their diets, too.

The take-home message is that, while there were certainly some differences in jaw mechanics between the major groups of plant-eating dinosaurs, we don’t see the same subtle differences between closely related species within those groups as we see in today’s Galápagos finches.

This tells us that the Albertan plant-eating dinosaur communities operated in a slightly different way, and that perhaps differences in other aspects of their anatomy allowed these animals to exploit different food sources to coexist.

Some of my previous work suggests that this is the case—we do see many differences in skull and beak shapes, feeding heights and tooth wear, even among closely related species. These insights tell us something about what it takes to support a diverse ecosystem over long periods of time—something that concerns conservationists today. It’s amazing what can be learned by plumbing the depths of our vast collections!

Read an abstract of the article: http://www.tandfonline.com/doi/full/10.1080/02724634.2014.904323#abstract.

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2 Responses to Jawing about Dinosaurs: What the Herbivore’s Jaws Are Telling Us

  1. Duane Nash says:

    Personally I feel the importance of niche partitioning via anatomical features in dinos is and has been overstated. First we need to prove a resource is limiting, to my knowledge this has not been demonstrated in Alberta or any other formations. Secondly even in modern ecosystems posited as examples of niche partitioning among large herbivores, those in East Africa usually mentioned specifically, there is often considerable overlap in diet. And the herbivores coexist anyways. Often times partitioning does not occur through a suite of anatomical features – but resources are partitioned on a landscape scale. Some species preferring canopy, forested environs; others in open spaces; some liking more edge habitat. Perhaps, and what I suspect, this is what was going on in many dinosaur dominated ecosystems. Some amount of niche partitioning due to anatomical difference but often considerable overlap in diet often the case – but dinos inhabiting different habitats across the landscape, perhaps even seasonally.

    Duane Nash antediluviansalad.blogspot

  2. Jordan Mallon says:

    Duane, thanks for the thoughtful comments. I agree wholeheartedly with them. As you know, the focus of much of my recent research has been on the role of dietary niche partitioning, but I’m happy to oblige that other means of niche partitioning — be it spatial or temporal — may also have played a role in facilitating dinosaur diversity. The problem is that these are more difficult (but not impossible) suss out with the fossil record; they also tend to play a subsidiary role to dietary niche partitioning in modern ecosystems. I’m actually playing with some data now on spatial distribution that will hopefully create a more complete picture of what’s going on in Late Cretaceous Alberta.
    Re: the role of limiting resources, I think your comments are on point. I’d simply point out that resources tend to be limiting in modern ecosystems — either because they are inaccessible or cannot be readily digested — and I think it’s parsimonuous to suspect that the same is true of the analogous Late Cretaceous flora of Alberta. Regardless, the biostratigraphic data for the Dinosaur Park Formation seem to suggest that no two dinosaurs of like ecomorphotypes lived in sympatry (at least not commonly), which I think is at least indicative of resource limitation. More on that soon.

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