The discovery of new fossils of cynodonts from southern Brazil by a team of paleontologists from the University of Bristol, together with colleagues from Argentina and Brazil, led to a significant advance in our understanding of mammal evolution.
These fossils, belonging to the precursor species Brasilodon quadrangularis and Riograndia guaibensis, offer a critical view on the development of the jaw and the middle ear in mammals, revealing evolutionary experiments that occurred millions of years earlier than previously thought.
Mammals stand out among vertebrates for their distinctive jaw structure and the presence of three middle ear bones. This transition from earlier vertebrates, which had a single middle ear bone, has long fascinated scientists.
The study explores how the ancestors of mammals, known as cynodonts, developed these characteristics over time.
Using computed tomography, the researchers were able to digitally reconstruct, for the first time, the jaw joint of these cynodonts.
They found a “mammal-like” contact between the skull and the lower jaw in Riograndia guaibensis, a cynodont species that lived 17 million years before the previously oldest known example of this structure, but they did not find it in Brasilodon quadrangularis, a species closer to mammals.
This indicates that the feature that defines the mammalian jaw evolved several times in different cynodont groups, earlier than expected.
Evolution that was much more complex and varied than previously thought
These results suggest that the ancestors of mammals experimented with different jaw functions, leading to the evolution of “mammalian” features in a mosaic across several cynodont lineages. The initial evolution of mammals, it seems, was far more complex and varied than previously thought.
The lead author, James Rawson, from the Bristol School of Earth Sciences, explains that the “acquisition of the mammalian jaw contact was a key moment in mammal evolution”.
“What these new Brazilian fossils showed is that different groups of cynodonts were experimenting with various types of jaw joints, and that some features once considered exclusively mammalian evolved numerous times also in other lineages,” he adds.
This discovery has broad implications for understanding the initial stages of mammal evolution, illustrating that features such as the jaw joint and the mammalian middle ear bones evolved in a patchwork, or mosaic, across different cynodont groups.
The article’s co-author, Pam Gill, Scientific Associate of the Natural History Museum in London and the University of Bristol, notes that “this study was made possible thanks to the specimens of three fundamental cynodont taxa, Brasilodon, Riograndia and Oligokyphus, which were digitized to reconstruct the jaw articulation. The exciting recent specimens of Brasilodon and Riograndia from Brazil provided new information, but it was also important to digitize specimens of Oligokyphus from the Natural History Museum’s collections.”
“Although Oligokyphus was initially described in the 1950s, these recent detailed digitizations confirmed the earlier interpretations of the jaw joint structure, making it possible to include them with confidence in the analysis,” she explains.
Agustín Martinelli, from the Museo Argentino de Ciencias Naturales in Buenos Aires, adds that “in recent years, these tiny fossil species from Brazil have brought wonderful information that enriches our understanding of the origin and evolution of mammalian characteristics. We are only at the beginning, and our multinational collaborations will bring more news soon.”
The research team is eager to continue studying the South American fossil record, which has proven to be a rich source of new information about the evolution of mammals.
Professor Marina Soares of the National Museum, Brazil, stated that “nowhere else in the world is there such a large variety of cynodont forms, closely related to the earliest mammals.”
By integrating these findings with existing data, the researchers hope to deepen their understanding of how the earliest jaw articulations functioned and contributed to the development of the mammalian form.
James concludes: “the study opens new doors to paleontological research, as these fossils provide invaluable evidence of the complex and varied evolutionary experiments that ultimately gave rise to modern mammals.”
