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Eomaia scansoria is a newly discovered fossil of a mouse-sized mammal either on or close to the ancestral line of all placental mammals. It is 125 million years old and far better preserved than any previously known placental. Before this we had teeth from 110 million years ago and skeletons from only 75 million years. This Eomaia has an almost complete skeleton, and both hair and fur are clearly distinguishable.

It was found in the fossil-rich Yixian beds in China's Liaoning province, and has just been published in Nature (vol. 416, p. 816).

It is eutherian, the group that includes placentals, but is not itself placental. It has an epipubic bone, which supports the young in the pouches of marsupials but does not occur in modern placentals, and it has a narrow pelvis, so could not have borne very large young.

The well-preserved finger and toe bones and the proportions of its limbs and claws show that it was agile both on the ground and in bushes. It resembles a modern shrew, about 10 cm long, or about 15 cm including tail, and would have weighed perhaps 25 g. Its teeth suggest it ate insects.

The name is from the Greek eo- meaning dawn, and Maia, a minor Roman earth-goddess or in Greek mythology the daughter of Atlas and mother of Hermes, though press reports have just said it means 'mother'. Scansoria means climber in Latin.

Photograph and a digest of the Nature report: http://www.nature.com/nsu/020422/020422-15.html

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