Evrim Yazgin
Cosmos science journalist
Fossils found in Brazil are leading palaeontologists to re-write the evolution of mammals.
CT scans of mammal-precursor species Brasilodon quadrangularis and Riograndia guaibensis reveal that uniquely mammalian features in jaws and inner ears developed millions of years earlier than previously thought.
The findings are detailed in a paper published in Nature.
Mammals’ jaw structure and presence of 3 middle ear bones are unique among vertebrates. This transition from proto-mammal ancestors which had 1 middle ear bone has long been an area of interest for palaeontologists.
The earliest mammal ancestors – like Brasilodon and Riograndia – are called cynodonts. This group of creatures emerged during the Triassic period (252–201 million years ago) – the first epoch in the “Age of Dinosaurs”.
It was previously believed that mammalian jaw features evolved just before the end of the Triassic. But the new research shows mammalian forms evolved much earlier.
Researchers were able to digitally reconstruct the jaws of Brasilodon and Riograndia for the first time using their CT scans. Both animals lived about 225 million years ago and were discovered in the Caturrita Formation in far-south Brazil.
The palaeontologists found no sign of mammalian features in the jaw of Brasilodon.
However, they were surprised to find ‘mammalian-style’ contact between the skull and lower jaw in Riograndia. This is 17 million years earlier than the previous oldest example of such a structure in the fossil record.
“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,” says lead author James Rawsom from the UK’s University of Bristol.
It suggests early mammal evolution was more complex than previously thought.
“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,” says co-lead author Agustín Martinelli, from the Museo Argentino de Ciencias Natural of Buenos Aires, Argentina.
“We are just in the beginning and our multi-national collaborations will bring more news soon.”
South America is fast becoming a central location for the study of early mammal evolution.
“Nowhere else in the world has such a diverse array of cynodont forms, closely related to the earliest mammals,” adds co-author Marina Soares of the Museu Nacional, Brazil.
