Placental and marsupial mammals are more closely related to one another than to the third living group of mammals, the monotremes. These exotic egg-laying animals are represented by just three species: the duck-billed platypus and two echidnas.
"Looking across the world, 99.9 percent of modern mammals are placentals and marsupials, so finding how they came about is an important question," said Luo. "Establishing the origin of these two groups depends on determining the ancestral condition of the earliest fossil." The discovery of Sinodelphys brings that goal one step closer to reality.
Chinese co-author Qiang Ji, a paleontologist of the Chinese Academy of Geological Sciences in Beijing and discoverer of many important Liaoning fossils, acquired the fossil from Chinese peasants in 2000. Ji was on a field trip to fossil-bearing sites of Liaoning Province, 200 kilometers (120 miles) northeast of Beijing. These sites have yielded the remains of birds, frogs, some feathered dinosaurs and a few mammals including Eomaia, also 125 million years old.
Digging for fossils is more lucrative for locals than traditional subsistence farming. Luckily most of the important specimens have ended up in the collections of Chinese research institutions rather than dealers or private collectors, said Luo, adding that fossils from these sites should be better protected.
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