The earliest known mammals — dated at about 110 million years (Myr) — have been identified previously from small numbers of fossil teeth, making behavioral and anatomical comparisons difficult. In 25 April Nature, Ji Qiang and colleagues at the Chinese Academy of Geological Sciences, Beijing, report the discovery of an almost intact fossil eutherian mammal that is 125 Myr old (Nature 2002, 416:816-822).

Qiang et al. identified Eomaia scansoria (Eomaia meaning 'Dawn Mother' and scansoria denoting its probable climbing behavior) from the Yixian Formation of Liaoning Province, China, that has yielded insects and feathered dinosaurs in the past. From phylogenetic analysis they conclude that, although it is a primitive placental mammal, it probably reproduced like a marsupial. The presence of forward-extending epipubic bones and a narrow pelvic girdle suggests a short gestation period followed by parental nurture of young suspended from the abdomen.

In addition, analysis of the limbs of the fossil showed elongated toe and finger bones, suggesting that it was adapted to grasping twigs. The authors conclude, "the earliest eutherian lineages developed different locomotory adaptations, facilitating their spread to diverse niches in the Cretaceous."

"Molecular and fossil evidence indicate that both lineages [marsupial and placental] diversified considerably during the Cretaceous…It seems at least possible that a propensity to climb gave them the upper hand," said Anne Weil of Department of Biological Anthropology & Anatomy, Duke University, in an accompanying News and Views article.