The role of the rolling jaw in early mammals

A new study has given scientists more to chew on regarding the evolution and inner workings of the mammalian jaw.

Researchers at Yale, Brown, Harvard, and the University of Texas-Austin found two key characteristics of how early mammals chewed their food: they had a “rolling jaw” that offered more flexibility, and they used this flexibility to produce a grinding motion with their back teeth.

We mammals live hot and fast. Our metabolisms burn a lot of fuel in service to our large brains and warm bodies, and we maintain a heightened fuel intake by processing food more efficiently that our ancestors did. We start the digestive process in the mouth instead of in the stomach,” said Yale paleontologist Bhart-Anjan Bhullar, corresponding author of the new study published Feb. 13 in the journal Nature.

Whereas reptiles swallow prey whole using gravity, mammals chew to break food into small pieces and then swallow it using muscles designed specifically for that purpose. Although it may appear to be a simple procedure, efficient chewing and swallowing requires a delicately balanced set of tools — mobile jaws and tightly interlocking teeth — and a precisely coordinated set of motions.

A lizard and a leopard looking at similar prey — one swallows food whole, the other can chew.
Researchers have found that a “rolling jaw” was a key characteristic of how early mammals chewed their food, a characteristic still found in some mammals today. (Illustration: Michael Hanson)

Mammalian chewing and the accompanying changes to teeth and jaws have traditionally been considered the great leap forward for mammals, key to their early success and rise to dominance after the extinction of the dinosaurs,” Bhullar said. “Prior to our study, the dynamics of chewing were known only in highly modified, specialized mammals. We understood little about the way in which chewing worked at its origin, and therefore little about the actual nature of this major evolutionary innovation.”

It should be noted that humans do not have rolling jaws, because our lower jaws are fused at the chin. The human chewing motion involves moving the entire mandible from side to side.

Yet the only detailed studies of chewing dealt with mammals that chew by moving their jaws together the same way humans do. In part, the researchers said, this is because jaw rolling is difficult to detect using traditional 2D x-ray techniques.

The new findings are based on a 3D technology called X-ray Reconstruction of Moving Morphology (XROMM) that re-creates the movement of bones in 3D videos. In this case, the team used XROMM to study mammalian chewing motions of the Short-Tailed Opossum, which has the same ancestral chewing stroke as early mammals.

The researchers found that a rolling of the jaw is essential for positioning molars in the back of the jaw, and also for positioning the slicing premolar teeth in the front of the jaw. In addition, the researchers discovered that this system for aligning teeth coincided with another jaw feature onto which the muscles that roll the jaw will attach.

We think that jaw rolling and mortar-and-pestle grinding of food in the molars were linked, beginning early in mammalian evolution,” Bhullar said.

Armita Manafzadeh of Brown University is the study’s co-first author, with Bhullar. Additional authors are Juri Miyamae of Yale, Eva Hoffman of Harvard and the University of Texas-Austin, Catherine Musinsky and Alfred Crompton of Harvard, and Elizabeth Brainerd of Brown.

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Jim Shelton: james.shelton@yale.edu, 203-361-8332