Scientists identified the earliest known mammal by its teeth

Brasilodon is the oldest extinct vertebrate with two successive sets of teeth.
Loukia Papadopoulos
The oldest extinct mammal.
The oldest extinct mammal.

Natural History Museum 

The fossil dental records of the oldest known mammal - Brasilodon quadrangularis - have been identified, according to a press release by the Natural History Museum. The two sets of teeth come from a small ‘shrew-like’ animal that measured around 20cm in length.

225 million years ago

The dental records date from 225 million years ago (Late Triassic/Norian), 25 million years after the Permian-Triassic mass extinction event that led to the extinction of roughly 70 percent of terrestrial vertebrate families. Researchers from the Natural History Museum in London King's College London made the discovery and the Federal University of Rio Grande do Sul in Porto Alegre.

Dr. Martha Richter, Scientific Associate at the Museum and senior author of the paper, said: “Comparative studies with recent mammal dentitions and tooth replacement modes suggest that this was a placental, relatively short-lived animal. Dated 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 this period and the evolution of modern mammals.”

Scientists relied on clues from fossils of hard tissues such as bones and teeth because mammalian glands have not been preserved in any fossils found. Until this discovery, the Morganucodon had been considered the first mammal dating back around 205 million years.

On the other hand, Brasilodon is the oldest extinct vertebrate with two successive sets of teeth, including only one set of replacements, also known as a diphyodonty. This differs from that of reptiles who regenerate new teeth multiple times during their lives, the ‘many for one’ replacement also known as poliphyodonty.

Diphyodonty explained

Diphyodonty involves profound, time-controlled changes to the skull anatomy, for instance, the closure of the secondary palate (the roof of the mouth) that allows the young to suckle while breathing simultaneously. It has also been shown to be linked to endothermy and even placentation (live birth) and fur.

Brasilodon was the oldest known dinosaurs and probably lived in burrows like the shrews today.

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Prof Moya Meredith Smith, contributing author and Emeritus Professor of Evolution and Development of Dentoskeletal Anatomy at King’s College London, said: “The evidence from how the dentition was built over developmental time is crucial and definitive to show that Brasilodons were mammals. Our paper raises the level of debate about what defines a mammal and shows that it was a much earlier time of origin in the fossil record than previously known.”

The new research pushes back the origin of diphyodonty in Brasilodon and mammals by 20 million years.

Richter concluded: “This research is a collaboration between Brazilian and British scientists, who brought together their expertise on skull development, dental anatomy, physiology and histology to interpret the juvenal and adult fossils of the extinct species Brasilodon quadragularis.”

The paper Diphyodont tooth replacement of Brasilodon - a Late Triassic eucynodont that 2 challenges the time of origin of mammals is published in the Journal of Anatomy.

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