Fossils discovered in Brazil shed light on mammal evolution

A new study featured in the journal Nature, sheds light on the evolution of mammalian jaw and middle ear bone traits.

brazilian cynodonts
Artists depiction of Riograndia and Brasilodon. Credit: Jorge Blanco

A new study published in Nature describes two mammal-precursor species called Brasilodon quadrangularis and Riograndia guaibensis. These fossils offer insight into the development of the mammalian middle ear and jaw, revealing evolutionary characteristics that occurred millions of years earlier than previously hypothesised.

Evolution of the jaw and middle ear bones

Mammals have distinct jaw structures and the presence of 3 middle ear bones. This transition from earlier vertebrates – animals with backbones – has fascinated researchers as earlier vertebrates only had 1 ear bone. The new study describes how cynodonts, the ancestors of mammals, evolved these features.

The researchers created CT scans of the fossils to reconstruct the jaw joints of these cynodonts for the first time. They discovered a mammalian style contact between the skull and the lower jaw in Riograndia guaibensis, a species of cynodont that lived 17 million years before the previously known animal to have this structure. However, they did not find this structure in Brasilodon quadrangularis, which is a species that is more closely related to mammals. This discovery suggests that defining mammalian jaw features would have evolved multiple times in different groups of cynodonts, and much earlier than expected.

The findings suggest that the ancestors of mammals would have experimented with different jaw functions, which would have led to the evolution of ‘mammalian ‘traits’ independently in different lineages. This shows that the early evolution of mammals was far more varied and complex than previously thought.

Lead author James Rawson from Bristol’s School of Earth Sciences explained: "The acquisition of the mammalian jaw contact was a key moment in mammal evolution. What these new Brazilian fossils have shown is that different cynodont groups were experimenting with various jaw joint types, and that some features once considered uniquely mammalian evolved numerous times in other lineages as well.”

'Mosaic' evolution

The study's findings have broad implications for the understanding of early mammal evolution, suggesting that features such as the mammalian middle ear and jaw joint evolved in a mosaic fashion across different cynodont groups.

Dr Agustín Martinelli, from the Museo Argentino de Ciencias Natural of Buenos Aires, said: “Over the last years, these tiny fossil species from Brazil have brought marvellous information that enrich our knowledge about the origin and evolution of mammalian features. We are just in the beginning and our multi-national collaborations will bring more news soon.”


The research team wishes to continue studying the South American fossil record, which has provided a rich source of new information on mammal evolution.

Professor Marina Soares of the Museu Nacional, Brazil, said: “Nowhere else in the world has such a diverse array of cynodont forms, closely related to the earliest mammals.”

By combining this study’s findings and existing data the research team hopes to deepen the understanding of how early jaw joints functioned.

James added: “The study opens new doors for paleontological research, as these fossils provide invaluable evidence of the complex and varied evolutionary experiments that ultimately gave rise to modern mammals.”

Source of the news:

Rawson, J.R.G., Martinelli, A.G., Gill, P.G., Soares, M.B., Schultz, C.L. and Rayfield, E.J. (2024). Brazilian fossils reveal homoplasy in the oldest mammalian jaw joint. Nature. [online]