Tiny fossils reveal how reduction was essential for successful evolution

A new study published today in Nature shows that size reduction was a key factor contributing to the exceptional evolution of mammals over the last 200 million years.

The origin of modern mammals goes back more than 200 million years, until the age of the dinosaurs. But as dinosaurs evolved into some of the largest terrestrial animals, the ancestors of all modern mammals adopted, for the next 150 million years, a totally different strategy: to become very small.

An international team of scientists from the United Kingdom and the United States used modern computer analysis to examine what happened to the skeleton of our ancestors of small mammals.

Modern mammals have the peculiarity of having a lower jaw made up of a single bone that bears teeth. In contrast, all other vertebrates have complex lower jaws formed from at least five or more bones joined together. During evolution, the fossils show that the lower jaw of the ancestors of mammals has been simplified and that a new jaw joint has formed, while some of the other bones have moved in the middle ear to facilitate hearing.

The team's research focused on how it was possible to simplify and restructure the lower jaw, while being able to feed and hear. Using computed tomography (CT) on several fossil skulls and lower jaws, the researchers generated numerical models that were subjected to different computer simulations.

Their results showed that the small size of fossil mammals significantly reduced jaw bone stress during feeding, while being powerful enough to capture and bite prey, such as insects.

Dr. Stephan Lautenschlager, lead author and lecturer at the University of Birmingham, said, "Our findings provide a new explanation of how the jaw of mammals evolved more than 200 million years ago. they reduced jaw stress during feeding and made possible the restructuring of the jaw bones. "

Professor Emily Rayfield of Bristol University, who led the study, added, "The evolution of the jaw joint of mammals has puzzled paleontologists for more than 50 years. co-opting bones in a complex sound detection system. Our research is aimed at testing ideas about what makes mammals unique in the animal kingdom and how this may have happened. "

"The role of miniaturization in the evolution of the jaw and the middle ear of mammals" by Stephan Lautenschlager, Pamela Gill, Zhe-Luo Xi, Michael J. Fagan and Emily Rayfield is published in Nature.

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More information:
The role of miniaturization in the evolution of the jaw and the middle ear of mammals Nature (2018). DOI: 10.1038 / s41586-018-0521-4, https://www.nature.com/articles/s41586-018-0521-4