Tiny fossils reveal how shrinking was essential for successful evolution

Life reconstruction of a Morganucodon

Life reconstruction of the Morganucodon, found in Jurassic sediments of Wales and China. The Morganucodon was one of the smallest fossil mammal ancestors with a size of 4-6cm.

18 September 2018 by Dr Stephan Lautenschlager

Scientists have discovered that mammals successfully evolved over the years by getting smaller in size.

The origin of modern mammals can be traced back more than 200 million years to the age of dinosaurs. But while dinosaurs evolved to become some of the largest land animals, for the following 150 million years the ancestors of all modern mammals pursued an entirely different strategy - getting very small.

Modern mammals are unique in having a lower jaw consisting of just a single bone that bears teeth. In contrast, all other vertebrates possess complex lower jaws formed by at least five or more bones joined together. In the course of evolution, fossils show that the lower jaw of mammalian ancestors became simplified and a new jaw joint was formed, while some of the other bones moved into the middle ear to aid in hearing. The team of scientists, from the UK and the US, wanted to find out how it was possible for mammals to simplify and restructure their lower jaws, while maintaining the ability to feed and hear. Using modern computer analysis to scan fossil skulls and lower jaws, they looked at what happened to the skeleton of our tiny mammal ancestors.

Their results showed that the small size of the fossil mammals significantly reduced the stresses in the jaw bones when feeding, while still being powerful enough to capture and bite through prey, such as insects.

Dr Stephan Lautenschlager, lead author and lecturer at the University of Birmingham, explained:

Our results provide a new explanation of how the mammalian jaw evolved over 200 million years ago. 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.

Professor Emily Rayfield from the University of Bristol who led the study added:

The evolution of the mammalian jaw joint has perplexed palaeontologists for over 50 years. 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. Our research is about testing ideas of what makes mammals unique among the animal kingdom, and how this may have come about.

The NERC-funded research was carried out at the University of Bristol. Research paper 'The role of miniaturisation in the evolution of the mammalian jaw and middle ear' was published in Nature on 17 September 2018.