These fossils, belonging to the mammal-precursor species Brasilodon quadrangularis and Riograndia guaibensis, offer critical insights into the development of the mammalian jaw and middle ear, revealing evolutionary experiments that occurred millions of years earlier than previously thought.
Mammals stand out among vertebrates for their distinct 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 new study explores how mammal ancestors, known as cynodonts, evolved these features over time.
Using CT scanning, researchers were able to digitally reconstruct the jaw joint of these cynodonts for the first time. The researchers uncovered a 'mammalian-style' 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 did not find one in Brasilodon quadrangularis, a species more closely related to mammals. This indicates that the defining mammalian jaw feature evolved multiple times in different groups of cynodonts, earlier than expected.
These findings suggest that mammalian ancestors experimented with different jaw functions, leading to the evolution of 'mammalian' traits independently in various lineages. The early evolution of mammals, it turns out, was far more complex and varied than previously understood.
Lead author James Rawson based in 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."
This discovery has broad implications for the understanding of the early stages of mammal evolution, illustrating that features such as the mammalian jaw joint and middle ear bones evolved in a patchwork, or mosaic, fashion across different cynodont groups.
The research team is eager to further investigate the South American fossil record, which has proven to be a rich source of new information on mammalian evolution.
Professor Marina Soares of the Museu Nacional, Brazil, stated: "Nowhere else in the world has such a diverse array of cynodont forms, closely related to the earliest mammals."
By integrating these findings with existing data, the scientists hope to deepen their understanding of how early jaw joints functioned and contributed to the development of the mammalian form.
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."
Paper:
'Brazilian fossils reveal homoplasy in the oldest mammalian jaw joint' by James Rawson et al in Nature.
