How did early mammals chew?
At left, an ancient mammal relative. At right, a modern opossum. Both have distinct left and right jawbones, which allows chewing with a rotational bite. Illustration by Michael Hanson
Unless you have a toothache, it's easy to take chewing for granted. We crunch through foods like nuts and fruit without thinking much about the anatomical equipment that makes it happen. But we know from fossils that our earliest mammalian ancestors didn't have a single lower jaw bone like we do today; instead, they had left and right jaw bones that could move independently. How did early mammals chew? And how has mammalian chewing evolved over the past 250 million years? Recent research reveals clues.
Where's the evolution?
Sadly, we can't hop into a time machine and watch ancient mammals chomp down on tasty Jurassic bugs. When paleontologists want to reconstruct a behavior like chewing in a group of extinct animals, they often study the behavior in the group's living descendants. This experimental work can be especially informative when the animals share homologous traits — similarities in anatomy that result from common ancestry — that are involved in the behavior studied. So to answer questions about chewing in our mammalian ancestors, a team of researchers from Yale, Brown, and Harvard Universities turned to a living mammal — the short-tailed opossum, Monodelphis domestica. The researchers chose to work with opossums because they share two important homologous traits with early mammals: (1) separate left and right jaw bones, and (2) molars (back teeth) that are very similar to those of early mammalian fossils.
The researchers used a combination of X-ray videography and CT scanning to create 3-D animations of opossums' skull bones as they ate kibble and worms. They discovered that as opposums' mouths close, their lower jaw bones rotate their teeth in-and-out. This specialized "rotational grinding stroke" allows the upper and lower teeth to grind against each other like a mortar and pestle, chewing up food.
An opossum eating kibble. Colored lines show different chewing muscles. Credit: after Bhullar, Manafzadeh et al. 2019
So did ancient mammals chew rotationally like this? And how did chewing like ours, with a single lower jaw moving more simply up and down, evolve? Studying modern opossums provided important clues, but to piece it all together, the researchers turned to the evolutionary tree that shows how all mammals are related. They mapped the anatomical traits important for the rotational grinding stroke in living opossums, onto the evolutionary tree (shown below), and used this information to reconstruct key steps in the evolution of chewing.
Evolution of mammalian chewing, showing backs of jaws (left) and teeth (right) of different groups. Credit: after Bhullar, Manafzadeh et al. 2019
Ultimately, the team's analysis of fossil mammal jaws and teeth led them to conclude that the mortar-and-pestle rotational grinding stroke first appeared with the origin of a mammalian clade called Cladotheria, and was inherited by some modern lineages, including opossums. Like opossums, these early mammals also had a big grinding surface on their molars (called the talonid basin) and a bony protrusion on their jaws where the muscles involved in independent jaw rotation attach (called the angular process). In some mammalian groups, the two halves of the lower jaw fused and the teeth changed shape, and as this happened, those groups lost the ability to carry out the specialized rotational chewing motion and instead started chewing more like we do today – mostly straight up and down. So the next time you bite into a crunchy snack, take a moment to consider the hundreds of millions of years of evolution that went into that chomp!
Discussion and extension questions
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