Little Sinodelphys had it tough. The opossum-like creature lived in what is now China, where it was probably hunted by caudipteryx, a feathered, turkey-sized dinosaur with the nasty disposition of a raptor.
But Sinodelphys hung in there. Now, 125 million years later, the little guy is getting his due as the patriarch of all modern-day marsupials.
An article in today's issue of Science magazine gives props to Sinodelphys as the most primitive and the oldest known relative of the kangaroo, koala and other modern-day pouched mammals. Sinodelphys is nearly 15 million years older than previous contenders. Not bad for a 6-inch-long critter who tipped the scales at an ounce or so.
"This is really the earliest known and the most primitive known marsupial relative. This will help establish the whole evolutionary lineage," said Dr. Zhe-Xi Luo, curator of vertebrate paleontology at the Carnegie Museum of Natural History in Oakland.
Luo authored the article along with John Wible, curator of mammals at the Carnegie Museum of Natural History, and Dr. Qiang Ji of Nanjing University in China.
The fossil was discovered in 2000 in Liaoning province in northeastern China. Liaoning, whose terrain is dotted with shale quarries, has yielded a treasure trove of spectacularly preserved fossils, including feathered dinosaurs known as theropods; the earliest known relative of the flowering plant, archaeofructus ; and eomaia scansoria, the oldest known placental mammal.
The team determined that Sinodelphys was related to the marsupials by analyzing the bone structure in its wrist and ankles, said Luo, who lives in Franklin Park. The mammal's shoulder, limbs and feet indicated it probably was well adapted to climbing trees.
The Sinodelphys fossil not only had a nearly intact skeleton but impressions of soft tissue and fur. Researchers thus had more to go on besides its teeth.
"It's been a problem in particular because, prior to Sinodelphys, most everything that we knew was based on teeth," says Wible, of Ross. "We had all these criteria for distinguishing marsupials from placentals. But when we start getting closer to the common ancestry of these two groups, the teeth are becoming increasingly similar to each other, so it gets harder to tell them apart."
The name Sinodelphys derives from Sino for China and delphys , Greek for basal marsupial species. It dwelled in woods and shrubs near lakes and rivers and ate grubs and plants.
Placental mammals have a longer gestation period and give birth to more mature offspring. Marsupials produce much less-developed young, which they nourish in a pouch. This discovery is helping to determine when mammals split into those two different groups.
