Remarkable cynodont fossils from southern Brazil are helping researchers from the University of Bristol and their South American colleagues to better understand mammal evolution.  The fossils represent the mammal-precursors Brasilodon quadrangularis and Riograndia guaibensis.  These animals lived in the Late Triassic.  The fossils provide a critical insight into the development of the mammalian middle ear and jaw.  These key anatomical features of the Mammalia may have evolved millions of years earlier than previously thought.

Getting to Grips with Mammal Evolution

Mammals have a distinctive jaw structure and have evolved three middle ear bones to help with hearing.  The evolutionary transition from earlier tetrapods which had a single middle ear bone, has intrigued palaeontologists.  This new research, published in the journal “Nature” examines how mammalian ancestors (cynodonts), evolved these features over time.

Computed tomography (CT scans) was used to create digital models of the jaw joint of these Brazilian cynodonts.  The scientists discovered a “mammalian-style” contact between the skull and the lower jaw in Riograndia guaibensis.  This feature was not found in the Brasilodon quadrangularis fossil material.   B. quadrangularis is closer to the stem mammals than Riograndia guaibensis. This discovery suggests that the defining mammalian jaw feature evolved multiple times in different groups of cynodonts.  In addition, Riograndia lived around seventeen million years earlier than the previous oldest known example of this structure.  This indicates that this anatomical feature had evolved earlier than previously thought.

Riograndia guaibensis and Brasilodon quadrangularis life reconstruction. Brasilodon quadrangularis (left) and Riograndia guaibensis (right). Picture credit: Jorge Blanco.

Picture credit: Jorge Blanco

Brazilian cynodont fossils have played an important role in helping to better understand mammal evolution. In 2022, Everything Dinosaur reported on an analysis of the teeth of Brasilodon quadrangularis that led palaeontologists to suggest the Mammaliaformes (the lineage leading to the modern Mammalia), originated some twenty million years earlier than previously perceived.

To read this article: Nibbling Away at the Earliest Date for True Mammals.

New Study Suggests Mammaliaformes Experimented with Different Jaw Functions

The authors of this new study conclude that Mammaliaformes experimented with different jaw functions, leading to the evolution of “mammalian” traits independently in various lineages.

Lead author of the study, James Rawson (University of Bristol) 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 new research has significant implications for the understanding of the early stages of mammal evolution.  The study illustrates that features such as the mammalian jaw joint and middle ear bones evolved in a patchwork, or mosaic, fashion across different cynodont groups.

Fossils from Brazil Have Global Significance for Understanding Mammal Evolution

Co-author of the study, Dr Agustín Martinelli (Museo Argentino de Ciencias Natural, Buenos Aires) added:

“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 researchers are eager to investigate further the South American fossil record.  It has proven to be a rich source of new information on mammalian evolution.

Professor Marina Soares of the Museu Nacional, Brazil, exclaimed:

“Nowhere else in the world has such a diverse array of cynodont forms, closely related to the earliest mammals.”

By integrating these findings with the results from other studies, the scientists hope to deepen their understanding of how early jaw joints functioned and contributed to the development of the Mammalia.

James added:

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

Everything Dinosaur acknowledges the assistance of a media release from the University of Bristol in the compilation of this article.

The scientific paper: “Brazilian fossils reveal homoplasy in the oldest mammalian jaw joint” by James Rawson et al published in Nature.

The Everything Dinosaur website: Dinosaur and Prehistoric Mammal Toys.