The shrew-sized creature--described in the 25 April issue of Nature by Qiang Ji of the Chinese Academy of Geological Sciences in Beijing, Zhexi Luo of the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania, and colleagues--is called Eomaia, from the Greek for "dawn mother." It lived 125 million years ago, a time when the world was dominated by the far more varied and numerous dinosaurs.

Eomaia's age and characteristics place it near the base of the placental-mammal family tree. Features of its teeth place it in Eutheria, the group that includes all the living placentals as well as extinct mammals that are closer to placentals than to marsupials. "It really helps us link living placental mammals and extinct Mesozoic groups," says Guillermo Rougier, a paleontologist at the University of Kentucky, Louisville. When trying to figure out the relationships of placental orders, paleontologists need to know which anatomical traits came from ancestors and which are newly evolved--sometimes an impossible task when the most ancient eutherians were known only from teeth and jaws.

Eomaia also goes a little way toward closing a long-standing gap between fossil evidence and molecular dates for milestones in mammalian history. According to molecular geneticists, eutherians diverged from marsupials 170 million years ago. The latest molecular data also suggest that modern orders of mammals arose and began to diversify about 104 million years ago--some 40 million years before their undisputed fossil record begins. By showing that placental mammals had already begun diversifying by 125 million years ago, Luo says, his team's fossil meshes with the molecular evidence. But others say that because Eomaia doesn't belong to a modern order, it leaves the major discrepancy unchanged.

--ERIK STOKSTAD