A mouse-size, tree-climbing animal that lived with the dinosaurs is the oldest known ancestor of modern marsupial mammals, scientists say. A report published today in the journal Science describes the fossil, dubbed Sinodelphys szalayi, which is 15 million years older than the previous record holder.

Zhe-Xi Luo of the Carnegie Museum of Natural History (CMNH) and Nanjing University and his colleagues discovered the largely intact skeleton in China's Yixian rock formation, which dates to 125 million years ago. The find included well-preserved impressions of fur and carbonized soft tissue, which aided the reconstruction of what the animal looked like (see image). "This mammal could be the great grand aunt or uncle, or it could be the great grandparent of all marsupial mammals," Luo says. The creature was about 15 centimeters long and weighed about 30 grams, or one ounce. Its foot structure, in particular, indicates that it was capable of climbing trees.

"Interestingly, the more primitive mammals of the Yixian feathered dinosaur fauna were adapted to terrestrial or ground dwelling living," explains study co-author John R. Wible of CMNH. According to the team, this suggests that adaptations that favor climbing may have been important for the earliest divergence of marsupial lineage (whose living representatives include opossums, kangaroos and koalas) from the placental one. Because similarly aged fossils of placental mammals have been recovered from the same area, the scientists propose that the two groups originated in Eurasia. Richard L. Cifelli of the University of Oklahoma and Brian M. Davis of the Oklahoma Museum of Natural History, however, classify the issue in a related perspective as "open but ripe for testing through new discoveries."