Modern mammal ancestors managed to evolve into one of the most successful animal lineages. A new study reveals that the key was to start out modest and straightforward.

The cranium and lower jaw of animals with a backbone are composed of numerous bones in numerous vertebrate groups, including amphibians and reptiles. This was also true of the earliest mammalian ancestors over 300 million years ago.

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Between 150 and 100 million years ago, early animals lost skull bones due to evolution.

An international team of paleontologists published their findings today in Communications Biology. They describe how they used computer simulations and stress analyses to investigate the purpose of this cranium simplification.

As hypothesized for decades, their research demonstrates that reducing the number of cranium bones did not increase bite forces or skull strength. Instead, the team discovered that the cranium morphology of these early mammals efficiently redirected stresses during feeding.

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“Reducing early mammals’ skull bones redistributed stresses.”

Dr. Stephan Lautenschlager, Senior Lecturer for Palaeobiology at the University of Birmingham, stated, “Reducing the number of bones in the skulls of early mammals caused a redistribution of stresses. During nutrition, stress was redirected from the portion of the cranium containing the brain to the skull’s margins, which may have allowed for an increase in brain size.

“Changes in skull structure and the diminution of mammalian size are associated with a switch to an insect-based diet, allowing the subsequent diversification of mammals that led to the evolution of the wide variety of creatures we see today.”

In addition to the reduction of cranial bones, the study revealed that the size of early mammals shrunk significantly, with some possessing a cranium length of only 10–15 mm. This reduced the available food sources significantly, and early mammals acclimated by consuming primarily invertebrates.

This combination of compact dimensions, reduced number of skull bones, and grazing on novel food sources such as insects enabled the ancestors of modern mammals to flourish in the shadow of the dinosaurs.

However, it was not until the end of the Cretaceous, approximately 66 million years ago, that mammals had the opportunity to diversify further and attain the wide spectrum of body sizes observed today.