The unearthing of new cynodont fossils from southern Brazil by a group of paleontologists has sparked a major advancement in our knowledge of mammal evolution.
These fossils, belonging to the early mammal-related species Brasilodon quadrangularis and Riograndia guaibensis, provide crucial information about the development of the mammalian jaw and middle ear, uncovering evolutionary changes that took place millions of years earlier than scientists previously believed.
Mammals distinctly differ from other vertebrates due to their unique jaw structure and the presence of three bones in the middle ear. The shift from earlier vertebrates that only had a single ear bone has long intrigued researchers. This new research delves into how the ancestors of mammals, called cynodonts, developed these traits over time.
By employing CT scanning technology, the research team was able to digitally reconstruct the jaw joint of these cynodonts for the first time. They discovered a ‘mammalian-style’ connection between the skull and the lower jaw in Riograndia guaibensis, a cynodont species that existed 17 million years earlier than the previously known oldest example of this structure. However, this connection was not found in Brasilodon quadrangularis, which is more closely related to mammals. This suggests that the key mammalian jaw characteristic evolved independently in several groups of cynodonts much earlier than anticipated.
These results indicate that the early ancestors of mammals explored various jaw functions, leading to the emergence of ‘mammalian’ traits across different lineages. The initial evolution of mammals appears to have been significantly more intricate and diverse than previously recognized.
Lead author James Rawson from Bristol’s School of Earth Sciences noted: “The development of the mammalian jaw connection was a pivotal event in the evolution of mammals.”
“What these new fossils from Brazil have revealed is that various cynodont groups were testing out different types of jaw joints, and features thought to be exclusive to mammals emerged multiple times in different lineages.”
This discovery has far-reaching consequences for our understanding of the early evolution of mammals, showcasing that traits such as the mammalian jaw joint and middle ear bones emerged in a patchwork or mosaic manner across various cynodont groups.
Dr. Agustín Martinelli of the Museo Argentino de Ciencias Naturales in Buenos Aires remarked: “In recent years, these small fossil species from Brazil have provided astonishing insights that enhance our understanding of the origins and evolution of mammalian traits. We are just scratching the surface, and our international collaborations will yield more discoveries soon.”
The research team is enthusiastic about exploring the fossil record in South America further, as it has turned out to be a valuable source of new information regarding mammalian evolution.
Professor Marina Soares from the Museu Nacional in Brazil commented: “No other place in the world boasts such a diverse range of cynodont forms that are closely linked to the earliest mammals.”
By combining these findings with existing knowledge, the scientists aim to gain a deeper understanding of how early jaw joints operated and how they played a role in the evolution of mammals.
James added: “This study paves the way for new avenues in paleontological research, as these fossils serve as crucial evidence of the diverse and complex evolutionary trials that eventually led to today’s mammals.”
