The earliest known mammal, identified by its fossilized teeth, was digging burrows while dinosaurs were still evolving

Researchers report the discovery of what is currently the oldest mammal ever discovered.

Wherever life takes us, it’s important to remember where we came from. And where we’re coming from seems to be the shrew, according to new research Brasilodon quadrangularis. This modestly sized animal (it could be up to 12 cm long by our estimates) is the oldest known mammal to date and has been described from a series of fossil dental records found in Late Triassic outcrops at Rio Grande do Sul, Brazil.

The rocks containing these fossils formed in the last days of the dinosaurs (during the Late Triassic) about 225 million years ago, about 25 million years after the Permian-Triassic mass extinction event, the most dramatic single extinction event in Earth’s known history.

Earliest Mammal

“Comparative studies with newer mammalian dentition and denture types indicate that it is a placental, relatively short-lived animal,” explains Dr. Martha Richter, Research Fellow at the History Museum in London, King’s College London and senior author of the publication.

“At 225.42 million years old, this is the oldest known mammal in the fossil record, contributing to our understanding of the ecological landscape of that time and the evolution of modern mammals.”

The current paper is the result of a collaboration between researchers at the museum and the Federal University of Rio Grande do Sul in Porto Alegre, Brazil.

The discovery shifts the age of the oldest known mammal by around 20 million years. The oldest known mammal to date was a species called Morganucodon, which is around 205 million years old.

Brasilodon, the newly described mammal, is the oldest known extinct vertebrate species with two consecutive teeth, one of which served as a replacement; This trait is known as diphyodontia. Humans are also diphyodont animals. Reptiles use a different mechanism called polyphydontia, in which their teeth are regenerated multiple times over their lifetime.

Diphyodonty is a telltale trait of mammals and has been linked to endothermy (warm-blooded animals), fur, and live birth. The development of this secondary dentition is associated with long-term and age-related changes in the anatomy of the animal’s skull, such as the closure of the palate, which allows the young to breathe during lactation.

Many of the traits that help define mammals—the presence of fur or specific milk-producing glands—very rarely fossilize. In the case of the current species, none of these could be recovered. However, the team was able to draw on the fact that Brasilodon was a diphyodont to determine that it was more of a mammal than a reptilian species, despite the comparative lack of other body structures.

“The evidence for how dentition was constructed over evolutionary time is crucial and definitive in demonstrating that Brasilodons were mammals,” explains Prof. Moya Meredith Smith, contributing author and Professor Emeritus of Evolution and Development of Dentoskeletal Anatomy at King’s College London.

“Our article increases the debate on what defines a mammal and shows that it was a much earlier origin in the fossil record than previously known.”

Based on its probable size and choice of neighborhood, Basilodon most likely led a similar lifestyle to today’s shrews. It lived at a time when the world was full of reptiles and many of the first species of dinosaurs were evolving, so it likely spent much of the day sheltering in burrows and going out at night to find food.

The publication “Diphyodont tooth replacement of Brasilodon – a Late Triassic eucynodont that 2 challenges the time of origin of mammals” was published in the Journal of Anatomy.