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An illustration of a eutherian mammal.

Juramaia sinensis, the earliest known eutherian mammal, walks along a tree branch.

Illustration courtesy Mark A. Klinger, Carnegie Museum of Natural History

John Roach

for National Geographic News

Published August 24, 2011

A tiny, shrew-like creature of the dinosaur era might have been, in a sense, the mother of us all.

Named the "Jurassic mother from China" (Juramaia sinensis), the newfound fossil species is the earliest known ancestor of placental mammals—animals, such as humans, that give birth to relatively mature, live young—according to a new study.

The 160-million-year-old specimen pushes back fossil evidence for the evolutionary split between the placental and marsupial lineages by 35 million years. Although it's unclear if the creature is a direct ancestor of modern placentals, it's "either a great grand-aunt or a great grandmother," the study authors say.

Placentals—including creatures from mice to whales—are all that remain of the so-called eutherian mammals, of which J. sinensis is the oldest known specimen.

The first eutherians evolved from the ancestors of marsupials, which have pouches and give birth to comparatively immature offspring. (A third type of mammal, the monotremes, includes platypuses and lays eggs.)

(See "Platypus Genome Reveals Secrets of Mammal Evolution.")

With forepaws adapted to climbing trees, the newfound eutherian scurried about temperate Jurassic forests feasting on insects under the cover of darkness. This diet allowed J. sinensis to tip the scales at around half an ounce (15 grams), making the creature lighter than a chipmunk.

"The great evolutionary lineage that includes us had a very humble beginning, in terms of body mass," said Zhe-Xi Luo, a paleontologist at the Carnegie Museum of Natural History in Pittsburgh, who led the team that discovered the fossil.

Lucky Breaks for Placentals and Marsupials

Eutherians arose from metatherians, the mammal lineage that led to marsupials such as kangaroos. (See: "Earliest Known Ancestor of Placental Mammals Discovered.")

Both lineages started out small and got lucky breaks when they moved up into the forest canopy, out of reach of most dinosaurs and other mammals that stalked the ground, Luo said.

"Once you get out on a tree, you have all the different ecological opportunities that weren't available for the terrestrial animals," he said.

The main physical differences between eutherians and metatherians are in their wrist bones and teeth. In particular, eutherians have fewer molars than metatherians.

J. sinensis' teeth "really jumped out" at Luo and indicated "it was a eutherian."

Placental Split Still a Mystery

The discovery brings the fossil record in line with DNA evidence, which had indicated that the split between ancestral marsupials and placentals occurred around 160 million years ago, Luo added.

What was happening during this epoch to force the evolutionary split, however, remains a mystery.

"What is clear is that—beside the fact that marsupials and placentals start to differentiate—we also have the other mammals that diversified as well," he said. "But we don't know what would be the specific environmental trigger for that."

The placental-mammal findings are to be published Thursday in the journal Nature.

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