for National Geographic News
Amateur fossil hunters have helped to uncover the oldest known ancestor to kangaroos, koalas, possums, and wombats. A near complete skeletal fossil of the chipmunk-size, marsupial ancestor Sinodelphys szalayi has been dug up from 125-million-year-old shales in China's northeastern Liaoning Province.
"This mammal could be the great-grandparent of all marsupial mammals," said Zhe-Xi Luo, one of the paleontologists behind the find, based at the Carnegie Museum in Pittsburgh. "This new fossil provides precious new information, and sheds light on the evolution of all marsupial mammals," he said.
Sinodelphys, which shares more wrist, ankle, and dental features with living marsupials than other mammals, will help researchers understand what the ancestor to all marsupials may have been like. The fossil will also help scientists piece together the early history of all placental and marsupial mammals, according to the Chinese and American paleontologists who detail their analysis of the fossil in the current edition of the research journal Science.
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.
Most archaic marsupial and marsupial-like fossils are known from North America, commented Mike Archer, vertebrate palaeontologist and director of the Australian Museum in Sydney. This discovery now shifts the limelight of the group's origins to Asia. "China is now regularly producing stunning fossils such as the world's oldest-known flower, Archaeofructus, from the same deposit as Sinodelphys," he said.
"[Sinodelphys] increases our appreciation for the antiquity of this great and fascinating group of mammals, and makes those of us who have obsessions about pouches proud to be so afflicted," said Archer.
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.
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