The fossils, described in the latest issue of the journal Nature, belonged to three newly named mammals that lived 160 million years ago in China.
Given the animals' relationship to other early known species, researchers now believe that mammals as a whole originated at least 208 million years ago during the Triassic. This corresponds with some other studies that used DNA data.
Most prior studies based on the fossil record, however, previously concluded that mammals originated during the middle Jurassic, between 176 and 161 million years ago.
"For decades, scientists have been debating whether the extinct group, called Haramiyida, belongs within or outside of Mammalia," co-author Jin Meng, a curator in the American Museum of Natural History's Division of Paleontology, said in a press release.
"Previously, everything we knew about these animals was based on fragmented jaws and isolated teeth," he continued. "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 three new species -- Shenshou lui,Xianshou linglong and Xianshou songae -- weighed only 1 to 10 ounces and had tails and feet that indicate they were tree dwellers. If you saw one in a tree today, you’d probably mistake it for a squirrel.
"They were good climbers and probably spent more time than squirrels in trees," Meng said. "Their hands and feet were adapted for holding branches, but not good for running on the ground."
They, and other members of their clade Euharamiyida, likely ate insects, nuts, and fruit with what the researchers say were "strange" teeth. Their teeth had many cusps, or raised points, on the crowns.
Mammals are thought to have evolved from a common ancestor that had three cusps; human molars can have up to five. The newly discovered species, however, had two parallel rows of cusps on each molar, with up to seven cusps on each side. No one knows yet how this complex tooth pattern evolved in relation to those of other mammals.
Read more at Discovery News
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