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When did mammals evolve? Fossils of an extinct squirrel-like animal reveal the answer

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Newly discovered fossils of an extinct animal reveal that the earliest mammals evolved faster and earlier than scientists thought. Fossils of an extinct squirrel-like animal were uncovered in China, suggesting that mammals originated more than 200 million years ago.

In a study published in the journal Nature, researchers reported finding six skeletons of an extinct squirrel-like mammal from three new species. The fossils change the timeline of mammal evolution, dating them back to when dinosaurs roamed the world.

The fossils of an extinct mammal called haramiyids were found in the past three years by private collectors and paleontologists in northeastern China. The fossils date back to the Triassic Period.

"For over a century, paleontologists were puzzled as to whether these creatures were mammals or animals closely related to mammals," American Museum of Natural History paleontologist Jin Meng says.

The creatures were identified by their distinct jaw and teeth that suggest "they were most likely omnivorous, eating insects, nuts and fruits."

Led by paleontologist Jin Meng, a team of researchers last year discovered the first full skeleton of the creature, which changes Mesozoic era evolution theories.

 "The picture that Mesozoic mammals were shrew-like insectivores that lived in the shadow of the dinosaurs needs to be repainted," says Meng, a co-author of the new study.

It was previously thought that the first mammals evolved during the Middle Jurassic period, 174 to 164 million years ago. However, the full skeletal remains reveal that the squirrel-like animals were in fact mammals and closely related to rodent-like creatures called multituberculates. The new evidence suggests that the mammals could have evolved "earlier than much previous research predicted."

"Haramiyids are one of the oldest group of mammals, if not the oldest," Meng says.

Meng and colleagues identified three previous unknown species of haramiyids, including Shenshou lui, which means "divine beast" and is named after collector Lu Jianhua.

The newly identified species had hands and feet that could grasp branches, evidence that suggests the haramiyids lived in trees above the dinosaurs.

However, any similarities between the early mammals and squirrels are only due to convergent evolution. The newly identified extinct species should not be compared to any living cousins.

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