extinct squirrels
Fossils of extinct squirrel-like species found in China.Zhao Chuang

Three previously undiscovered squirrel-like species from the Mesozoic era have placed the evolution of mammals at 208 million years ago, researchers have said.

Palaeontologists from the American Museum of Natural History and the Chinese Academy of Sciences have described three new species - Shenshou lui, Xianshou linglong, and Xianshou songae – after finding their fossils in China.

The squirrel-like species come from a poorly understood era of the mammal family tree. Their discovery gives support to the theory that mammals evolved in the late Triassic period, about 208 million years ago.

Published in the journal Nature, the three species were identified from six near complete fossils dating back 160 million years. They would have weighed between one and 10oz and had tails and feet that indicate they lived in trees.

"They were good climbers and probably spent more time than squirrels in trees," study co-author Jim Meng said. "Their hands and feet were adapted for holding branches, but not good for running on the ground."

Researchers believe the species ate nuts, insects, and fruit with "strange" teeth that had many raised points on the crowns, or cusps.

Their teeth are key to their importance. Mammals are believed to have evolved from a common ancestor that had three cusps – humans, for example, have five on their molars.

However, the new species' had two parallel rows of cusps, with seven on each side – a feature that did not fit in to other mammals from the time. However, other characteristics, including their ears, did fit with evolution models.

"Previously, everything we knew about these animals was based on fragmented jaws and isolated teeth. But the new specimens we discovered are extremely well preserved," Meng said. "And based on these fossils, we now have a good idea of what these animals really looked like, which confirms that they are, indeed, mammals.

"What we're showing here is very convincing that these animals are mammals, and that we need to turn back the clock for mammal divergence. But even more importantly, these new fossils present a new suite of characters that might help us tell many more stories about ancient mammals."