Researchers in China have discovered fossils which dramatically alter the story of mammalian evolution. The six fossils of tiny, squirrel-like creatures represent a new group called Euharamiyida.

Previously scientists only had teeth to represent the small creatures, which didn’t provide sufficient information.

“For decades scientists have been debating whether the extinct group, called Haramiyida, belongs within or outside of Mammalia. 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,” Dr. Jin Meng, at the American Museum of Natural History in New York, told the Daily Mail.

The animals, which have been named Shenshou lui, Xianshou linglong, and Xianshou songae, lived 160-170 million years ago during the Jarasic period. Because the animals were similar enough to have shared a common ancestor it puts the dawn of mammals approximately 208 million years ago during the late Triassic period. That date forces researchers to mark the dawn of mammals 38-40 million years earlier than previous estimates.

The newly discovered animals were about the size of a mouse, weighing from 1 to 10 ounces. They had long tails and feet that suggest that they spent more time in trees than modern squirrels. The Euharamiyida had unusual teeth with two rows of up to seven cusps on each molar and likely ate fruit, nuts and insects. Their three-boned middle ears suggest that their hearing was very much like that of modern mammals.

“What we’re showing here is very convincing that these animals are mammals and 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,” said Dr. Ming.

While this find is likely to raise more questions than it answers, it does help to provide a more complete picture of mammal and, ultimately, human evolution.

The findings are published in the journal Nature.