New Late Triassic Squirrels Discovered in China: Mammals Evolved Earlier Than Expected

Mammals may have appeared on our planet far earlier than thought. Paleontologists have uncovered three new small squirrel-like species that lived about 208 million years ago during the late Triassic.

"For decades, scientists have been debating whether the extinct group, called Haramiyida, belongs within or outside of Mammalia," said Jin Meng, co-author of the new study, in a news release. "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. 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."

The new fossils were discovered at a site in China and are called Senshou lui, Xianshou linglong and Xianshou songae. The animals probably looked similar to small squirrels, weighed between one and 10 ounces and had tails and feet that were probably used for climbing trees. In fact, it's likely that they actually spent more time in trees than modern day squirrels.

Previously, researchers believed that mammals diverged from reptiles at some point during the middle Jurassic, between 176 and 161 million years ago. Yet it appears that they appeared far earlier than that during the late Triassic. This actually corresponds with other studies that used DNA data.

After examining the fossils, the researchers found that these animals likely ate insects, nuts and fruits with their "strange" teeth, which consisted of many cusps on the crowns. The scientists also found evidence of a typical mammalian middle area, which is the area just inside the eardrum that turns vibrations in the air into ripples in the ear's fluids.

"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," said Meng. "But even more importantly, these new fossils present a new suite of characters that might help tell us many more stories about ancient mammals."

The findings are published in the journal Nature.