Stunning fossil is human's earliest mammal relative

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.

	Modern dormice also clamber nimbly through bushes (HPA)

Unusually, it reveals not only when placental mammals split from marsupials, but also how they lived. Eomaia, which means "ancient mother," 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 cm 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.

"A Mesozoic Pompeii"

"It's such a beautiful fossil, everybody who has seen it was absolutely stunned," says Zhe-Xi Luo of the Carnegie Museum of Natural History in Pittsburgh. He says debris from volcanic eruptions preserved the remains in "a Mesozoic Pompeii".

Having an entire skeleton gives palaeontologists insight into the animal's 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 "could be our great-great uncle or aunt 125 million years removed", 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's 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.

Journal reference: Nature (vol 416, p 816)

Jeff Hecht

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British Psychological Society