21 septembre 2014

Origine des mammifères


Les mammifères sont encore plus anciens que prévu:

Paleontologists have described three new small squirrel-like species that place a poorly understood Mesozoic group of animals firmly in the mammal family tree. The study, led by scientists at the American Museum of Natural History and the Chinese Academy of Sciences, supports the idea that mammals — an extremely diverse group that includes egg-laying monotremes such as the platypus, marsupials such as the opossum, and placentals like humans and whales — originated at least 208 million years ago in the late Triassic, much earlier than some previous research suggests.

(...) “For decades, scientists have been debating whether the extinct group, called Haramiyida, belongs within or outside of Mammalia,” said co-author Jin Meng, a curator in the Museum’s Division of Paleontology. “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 three new species – Shenshou lui, Xianshou linglong, and Xianshou songae – are described from six nearly complete 160-million-year-old fossils found in China. The animals, which researchers have placed in a new group, or clade, called Euharamiyida, likely looked similar to small squirrels. They weighed between 1 and 10 ounces and had tails and feet that indicate that they were tree dwellers.


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