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Mammals evolved to shrink in size and it was key to their success

Over the last 200 million years, mammals have thrived on this planet – and have done so through a unique evolutionary pattern. Mammals originated back when dinosaurs dominated the landscape. And while dinosaurs would evolve to become some of the largest land animals to ever exist, new research shows that mammals went the opposite route.

A recent study published in Nature reports that over the last 200 million years, mammals evolved to shrink in size. Using modern computer analysis to assess mammal skeletons, an international team of scientists focused in on the mammalian jaw.

Every species of vertebrate has a complex lower jaw formed by at least five bones joined together… except for modern mammals. The mammalian jaw is just a single bone that bears teeth. Through the course of evolution, fossils show that the lower jaw of mammalian ancestors became simplified, and formed a new jaw joint while also moving some bones to the middle ear to help with hearing.

The researchers wanted to know how it was possible for these past mammalian species to simplify and restructure their lower jaw, while still being able to feed and hear. Their study involved X-ray computed tomography (CT) scanning of many fossil skulls and lower jaws, which aided in generating digital models that they put through numerous computer simulations. The simulations showed that the smaller size of the mammals helped reduce stress in the jaw bones when they were eating, although they were still powerful enough to capture and kill prey – such as insects.

Our results provide a new explanation of how the mammalian jaw evolved over 200 million years ago,” says Dr. Stephan Lautenschlager, a lecturer at the University of Birmingham and lead author of the study. “Getting very small appears to have been crucial for our mammalian ancestors. This allowed them to reduce the stresses in the jaw during feeding and made the restructuring of the jaw bones possible.”

Mammals’ jaw joint evolution has been the topic of debate for decades, but this study may have just found the answer.

Using computational methods we can offer explanations to how our mammalian ancestors were able to maintain a working jaw while co-opting bones into a complex sound detection system,” says study leader Emily Rayfield, a professor at the University of Bristol. “Our research is about testing ideas of what makes mammals unique among the animal kingdom, and how this may have come about.”

By Connor Ertz, Staff Writer

Image Credit: Bob Nicholls,

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