午夜福利1000集合

OUR earliest mammalian ancestor was a dormouse-like creature that liked to rummage around in small shrubs. The tiny animal, discovered stunningly preserved in a Chinese lake bed, could fit in the palm of your hand. Unusually, it reveals not only when placental mammals split from marsupials, but also how they lived.

Eomaia, which means 鈥渁ncient mother,鈥� comes from the Yixian formation, the source of the famous feathered dinosaurs. For most early mammals all we have to go on are a few tiny teeth. But the nearly complete skeleton of Eomaia includes tiny hand and toe bones, plus a clearly recognisable coat of longer hair overlaying shorter fur.

About 16 centimetres long and 10 centimetres from nose to rump, Eomaia resembled a large dormouse. Its long fingers and claws could wrap around small twigs and grasp bark. Skeletal features show it was closer to modern placental mammals than to marsupials, so the two groups must have split before Eomaia came into existence about 125 million years ago. Before this discovery the oldest fossils of placental mammals were 110-million-year-old teeth, and the oldest skull and skeleton only around 75 million years old.

鈥淚t鈥檚 such a beautiful fossil, everybody who has seen it was absolutely stunned,鈥� says Zhe-Xi Luo of the Carnegie Museum of Natural History in Pittsburgh. He says debris from volcanic eruptions preserved the remains in 鈥渁 Mesozoic Pompeii鈥�.

Having an entire skeleton gives palaeontologists insight into the animal鈥檚 lifestyle, says Anne Weil of Duke University in North Carolina. The shape of the claws, its limb proportions and long fingers and toes show Eomaia had a highly specialised climbing ability, and was active both on the ground and in the lower reaches of bushes, says Luo, a member of the Chinese-American team that described the creature. Although Eomaia is not a direct ancestor of all placental mammals, it 鈥渃ould be our great-great uncle or aunt 125 million years removed鈥�, says Luo.

No soft tissue has been preserved, but the fossil bones suggest that unlike most modern mammals, Eomaia probably did not bear well-developed young nourished by a placenta inside the mother鈥檚 body: its narrow pelvis indicates the young were born quite small. Eomaia also has an epipubic bone, a structure that supports young in the pouches of modern marsupials but is missing in placental mammals.