Scientists have identified three new species of prehistoric squirrel-like mammals known as haramiyids that lived in China during the Jurassic period. The long-extinct species, which were about the size of mice but had long tails and feet suited for tree-dwelling, are some of the oldest mammals ever found, and their discovery pushes the timeline of mammalian evolution back by about 40 million years, according to a study published online Tuesday in the journal Nature.

Scientists have long suggested that mammals originated in the Middle Jurassic period between 174 million and 164 million years ago. The haramiyid fossils discovered in China are around 160 million years old, which means mammals would have had to originate much earlier – around 200 million to 220 million years ago, according to Live Science.

Researchers from the American Museum of Natural History and the Chinese Academy of Sciences discovered the fossils of six haramiyids in Liaoning province, northeast China, over the last three years. The fossils reveal new details about a group of animals that have been known since the time of Darwin but very little understood. Scientists have long debated whether the extinct group belongs within or outside the mammal clade or category.

"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," Jin Meng, a curator in the museum’s Division of Paleontology, said in a statement. "But even more importantly, these new fossils present a new suite of characters that might help us tell many more stories about ancient mammals."

Scientists named the three new species Shenshou lui, Xianshou linglong, and Xianshou songae. They weighed between 1 and 10 ounces each and probably ate insects, fruit and nuts.  

Previous haramiyid fossils had only turned up fragmented jaws and some teeth, according to Meng. But the new fossil finds were surprisingly intact and even included the ear bones, which help scientists distinguish mammals from other animals.