Starting small and simple - key to success for evolution of mammals

Newswise — The ancestors of modern mammals managed to evolve into one of the most successful animal lineages – the key was to start out small and simple, a new study reveals.

The skulls and lower jaws of animals with a backbone, such as fishes and reptiles, typically consist of multiple bones. This characteristic was also present in the earliest ancestors of modern mammals, dating back over 300 million years.

However, during evolution the number of skull bones was successively reduced in early mammals around 150 to 100 million years ago.  

An international team of paleontologists has published their research findings in Communications Biology, detailing how they employed computer simulations and stress analyses to explore the function of skull simplification in a particular species.

Their findings challenge the long-held hypothesis that reducing the number of skull bones would result in higher bite forces or increased skull strength. Contrary to previous assumptions, the team discovered that the simplified skull shape in these early mammals actually redirected stresses during feeding in a more efficient manner. The computer simulations and stress analyses revealed that the structural changes in the skull allowed for optimized distribution of forces, improving feeding efficiency without compromising skull strength. This groundbreaking research provides new insights into the evolutionary adaptations of these ancient mammals and sheds light on the complex interplay between form and function in skull evolution. The results of this study have important implications for our understanding of how different species have evolved and adapted to their environments over time.

Dr. Stephan Lautenschlager, Senior Lecturer for Palaeobiology at the University of Birmingham and lead author of the study, stated, "Reducing the number of bones led to a redistribution of stresses in the skull of early mammals. Stress was redirected from the part of the skull housing the brain to the margins of the skull during feeding, which may have allowed for an increase in brain size."

This insight from the research highlights how the simplification of the skull structure in these ancient mammals played a pivotal role in shaping their evolutionary trajectory. By redistributing stresses during feeding, the structural changes likely enabled the expansion of brain size, a critical factor in the evolution of mammals. The findings provide a novel perspective on the relationship between skull morphology, feeding behavior, and brain evolution in early mammals, contributing to our understanding of the complex interplay between form and function in the process of evolution.

“Changes to skull structure combined with mammals becoming smaller are linked with a dietary switch to consuming insects - allowing the subsequent diversification of mammals which led to development of the wide-range of creatures that we see around us today.”

Additionally, the study revealed that in conjunction with the reduction of skull bones, early mammals also underwent a significant decrease in size, with some species having a skull length as small as 10-12 mm. This miniaturization had profound implications for their feeding behavior, as it considerably limited the available food sources. As a result, these early mammals had to adapt to a diet primarily composed of insects.

The findings shed light on the close relationship between skull evolution, body size, and feeding ecology in early mammals. The miniaturization of the skull likely presented challenges in terms of accessing and processing food, which in turn drove adaptive changes in feeding behavior. The study provides valuable insights into the evolutionary strategies early mammals employed to cope with such constraints, highlighting the remarkable adaptability of these ancient creatures in the face of changing ecological conditions.

This combination of small size, reduced number of skull bones and feeding on new food sources, such as insects, allowed the ancestors of modern mammals to thrive in the shadows of the dinosaurs.

However, it was not until dinosaurs became extinct at the end of the Cretaceous, some 66 million years ago, that mammals had a chance to further diversify and reach the large range of body sizes seen today.

ENDS

Notes to Editors

• The University of Birmingham is ranked amongst the world’s top 100 institutions, its work brings people from across the world to Birmingham, including researchers and teachers and more than 8,000 international students from over 150 countries.
• ‘Functional reorganisation of the cranial skeleton during the cynodont–mammaliaform transition’ - Stephan Lautenschlager, Michael J. Fagan, Zhe-Xi Luo, Charlotte M. Bird, Pamela Gill and Emily J. Rayfield is published in Communications Biology.
• Participating institutions include the Universities of Birmingham, Hull and Bristol, in the UK; the University of Chicago, USA; and The Natural History Museum, London.