Skull simplification and miniaturization helped early mammals thrive

Many species of vertebrates, including fish and reptiles, possess skulls and lower jaws that are made up of multiple bones. This was also the case for the earliest ancestors of modern-day mammals over 300 million years ago.

However, throughout evolution, early mammals gradually reduced the number of bones in their skulls, a process that occurred roughly 150-100 million years ago.

In a recent publication in Communications Biology, a team of paleontologists from around the world detail their investigation into the reasons for this simplification. They utilized computer simulations and stress analyses to study the matter.

Contrary to the long-held hypothesis that reducing the number of skull bones would increase bite force and skull strength, the team’s research revealed that the skull shape of these early mammals instead allowed for more efficient redirection of stresses during feeding.

Digital skull model of the small-sized Jurassic mammal ancestor Hadrocodium wui. Credit: Dr Stephan Lautenschlager, University of Birmingham

Dr. Stephan Lautenschlager, the lead author and Senior Lecturer for Palaeobiology at the University of Birmingham, stated that the reduction of skull bones in early mammals led to a redistribution of stresses within the skull. This redistribution resulted in stress being redirected from the brain to the margins of the skull during feeding, which could have allowed for an increase in brain size.

Additionally, the study found that early mammals became smaller in size as they adapted to feed on insects. Some species had skulls as small as 10-12 mm, which limited their available food sources. However, this miniaturization, along with the reduction of skull bones and a shift towards insect-based diets, allowed the ancestors of modern mammals to survive alongside dinosaurs.

It was not until the extinction of dinosaurs at the end of the Cretaceous, roughly 66 million years ago, that mammals were able to diversify further and achieve the wide range of body sizes we see today.

Source: University of Birmingham

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