Study shows how mammal
jaws evolved to help our earliest ancestors eat a more diversified diet
Date: March 23, 2017
Source: University of
Chicago Medical Center
You probably haven't
given much thought to how you chew, but the jaw structure and mechanics of
almost all modern mammals may have something to do with why we're here today.
In a new paper published this week in Scientific Reports, David Grossnickle, a
graduate student in the Committee on Evolutionary Biology at the University of
Chicago, proposes that mammal teeth, jaw bones and muscles evolved to produce
side-to-side motions of the jaw, or yaw, that allowed our earliest ancestors to
grind food with their molars and eat a more diversified diet. These changes may
have been a contributing factor to their survival of the mass extinction at the
end of the Cretaceous Period 66 million years ago.
The terms
"pitch" and "yaw" usually describe movements of airplanes,
but biologists also use them to describe basic movements of body parts such as
the jaw. Pitch rotation results in basic up and down movement, and yaw rotation
results in side-to-side, crosswise motion (think of a cow munching away on some
grass). Almost all modern mammals, including placental mammals, like humans and
deer, and marsupials, like kangaroos and opossums, share similarities in their
jaw structures and musculature that allow for both pitch and yaw movements.
This allows mammals to have especially diverse diets today, from cutting pieces
of meat to grinding tough plants and vegetables. For early mammals, these
characteristics meant they could be more resourceful during tough times.
"If you have a very
specialized diet you're more likely to perish during a mass extinction because
you're only eating one thing," Grossnickle said. "But if you can eat
just about anything and 90 percent of your food goes away, you can still live
on scraps."
Using 2D images of early
mammal fossils from previous publications and 3D data collected from modern
specimens at the Field Museum, Grossnickle analyzed the structure of teeth, jaw
bones, and how the muscles that control them were attached to the skull. He saw
that as species began to develop a projection on the upper molars that fit into
a corresponding cup or basin on their lower counterparts, the musculature of
the jaw also changed to provide greater torque for side-to-side yaw movements.
This way the animal could grind its food between the molars like a mortar and pestle,
as opposed to cutting it with simple up and down pitch movements.
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