Researchers have unearthed the fossilized remains of what may be the mother of all placental mammals, so-named for the placenta that nourishes the young during gestation. The 125-million-year-old specimen is the earliest and most primitive known representative of the placental group, to which the vast majority of living mammals--humans among them--belong. Unlike other placentals known from the Cretaceous period, which exhibit adaptations to life on the ground, the newly discovered creature has features typical of climbers. As such it indicates that early placentals were a surprisingly motley crew. Discovered in the same quarry in northeastern China's Liaoning Province that previously yielded feathered dinosaurs, the fossil is also remarkable for its preservation: whereas most early mammal remains consist of just a few teeth or a jaw, the new find, dubbed Eomaia scansoria, is a nearly complete skeleton and even includes fur impressions.

Analysis of the creature's anatomy, conducted by Zhe-Xi Luo of the Carnegie Museum of Natural History (CMNH) in Pittsburgh and colleagues, revealed an agile, insect-eating, shrewlike beast with hands and feet built for grasping and branch-walking (see image). The team suspects that Eomaia was active both on the ground and in trees and shrubs, much as the opossum is. In fact, the ability to climb may have given early placentals a competitive edge by enabling access to food sources and refuges not available to their land-bound contemporaries, observes Anne Weil of Duke University in a commentary accompanying the report. She cautions, however, against generalizing about all early placentals on the basis of this one skeleton (the next oldest placental skeletons are some 50 million years younger than Eomaia). Weil further notes that primitive marsupials, the pouched mammals, were also climbers. Thus it may be that the common ancestor of these two groups had that ability. Whatever the case, "our new study," remarks team member John Wible, also at the CMNH, "shows that, in the Cretaceous, there was a far greater burst of diversity of extinct relatives of placentals than anyone had previously realized."