Scientists were wrong about a strange ancient mammal for 40 years

A remarkably complete fossil from Mongolia has overturned four decades of thinking about a group of small mammals that lived alongside the dinosaurs.

Researchers at the American Museum of Natural History in New York have shown that zhelestids, long thought to be early relatives of placental mammals, in fact belonged to a different group altogether. The study was published in Nature.

A new name

The fossil belongs to a species the team has named Tamirkhan balcarceli. It was found in the eastern Gobi Desert of Mongolia in 2004 and is the most complete zhelestid specimen discovered so far.

The animal measured between six and seven inches from head to tail. It had long, slender hind legs with distinctive ankles and long incisor teeth that grew continuously, like those of a modern rodent.

Misled by teeth

For about 40 years, palaeontologists grouped zhelestids with the ancestors of placental mammals, the group that today includes humans, whales, bats and most other mammals. That idea was based largely on their rounded teeth, which looked suited to eating plants.

With a much fuller skeleton to study, the team found that zhelestids belong instead to the zalambdalestoids, an extinct group. The rounded teeth, the researchers conclude, were probably an example of convergent evolution, where unrelated animals independently evolve similar features because they live in similar ways.

Why it matters

Teeth are often the only part of an ancient mammal that survives, so they are central to how scientists classify fossils. The finding is a reminder that teeth can mislead, and that a single well-preserved skeleton can reshape a family tree.

It also redraws part of the mammal family tree in the age of the dinosaurs, a period when small mammals were already diversifying into many different ways of life long before the dinosaurs died out 66 million years ago.

The study was led by Andres Giallombardo, with Eva Hoffman, Maureen O'Leary, Paul Velazco, Shawn Zack and Michael Novacek.