"Beautifully Preserved"

The near-complete skeleton of Sinodelphys is strikingly well preserved, with tufts of fur and even soft tissues imprinted on the slate slab in which it was found. In life, the slight, rodent-like marsupial would have measured six inches (15 centimeters) in length and weighed around an ounce (30 grams). The animal was a proficient climber and likely spent its days scampering across the low branches of trees and bushes, and feeding on insects, worms, and other invertebrates.

"To find such a beautifully preserved mammal of such antiquity is extraordinary … and this one will likely form the basis of debate for decades," commented marsupial fossil expert Steven Wroe at the University of Sydney in Australia. "The authors present a very convincing case for their conclusion that this new species is closely related to living marsupials," he said.

Though most marsupials are restricted to Australia today, all early fossil ancestors of the mammalian group are known from Asia and North America. The previous oldest known marsupial skeleton was unearthed from 75-million-year-old Mongolian deposits (though jaw fragments and teeth up to 105 million years of age have also been documented). The discovery of Sinodelphys follows the uncovering by the same research team, of the world's oldest placental mammal Eomaia scansoria last year. The discovery of both fossils was part funded by grants from the National Geographic Society.

To Pouch or Not to Pouch

Marsupial and placental mammals all share the habit of giving birth to live young. However the more than 4,300 placentals (such as humans, horses, elephants, and whales) have relatively long pregnancies and give birth to well-developed young. Marsupials on the other hand, have short gestation times, but further complete their maturation in their mother's pouch. The world's 270 marsupials mostly live in Australia, neighboring islands, and South America, with the exception of North America's Virginia opossum, a relatively recent immigrant.

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.