New research has used dental evidence to pinpoint when mammals first appeared.

This research was published in Journal of Anatomy.

The study, an international collaboration led by the Federal University of Rio Grande do Sul (UFRGS), in Porto Alegre, Brazil, which also included researchers from King’s College London and the Natural History Museum in London, UK, examined lower jaws of fossils of Brasilodon quadrangularisa mouse-sized animal that lived about 225 million years ago.

Analysis of the different growth stages showing tooth development in each of the fossils provided evidence that they were the remains of a mammal.

When did mammals appear?

Previously it was known that the time when mammals appeared was 205 million years ago. This new research suggests an earlier appearance of 20 million years.

“Evidence from how the dentition was built during development is crucial and definitive in showing that Brasilodon was a mammal. Our work ups the ante on the debate about what defines a mammal and shows an earlier time point of appearance in fossils than previously known,” says Moya Meredith Smith, from King’s College.

Mammary glands, which produce milk and feed young mammals today, have not been preserved in any fossils found to date. Scientists have therefore had to rely on “hard tissues”, mineralized bones and fossilizing teeth for alternative clues, writes

How are mammal teeth different?

Examining dentition found in Brasilodon quadrangularis fossils from southern Brazil and dated to about 225 million years ago (Late Triassic/Norian), the research team found evidence of a single set of permanent teeth.

This is a key characteristic of mammals, known as diphyodonty. Baby teeth begin to develop during the embryonic stage, and a second set (which is also the last) develops once the animal is born.

By contrast, reptilian dentitions are different, something called polyphyodonty, where each tooth will regenerate several times throughout a reptile’s life to replace damaged ones.

Milk and milk teeth

Diphyodontia is a complex and unique phenomenon that, along with the replacement of teeth, also involves profound, time-controlled changes in the anatomy of the skull, for example, the closure of the secondary palate (roof of the mouth), which allows the chick to suck and breathe at the same time .

“This research is a collaboration between Brazilian and British scientists who have brought together their expertise in cranial development, dental anatomy, physiology and histology to interpret the juvenile and adult fossils of the extinct species Brasilodon quadragularis,” says Dr Martha Richter, from the Museum of Natural History and main author of the paper.

Brasilodon existed at the same time as the earliest known dinosaurs and probably lived in burrows, like field mice today.

The new research takes the origin of diphyodonts in Brasilodon, and therefore mammals, back 20 million years and brings new information to the debate about when mammals first appeared.

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