Researchers Discover The Earliest Known Relative Of Marsupial Mammals

In an article published today in Science, the team of American and Chinese scientists describe a 125 million year old fossilized skeleton of Sinodelphys szalayi, ([Sino] - Latin for China, [delphys] - Greek term used for basal marsupial species; [szalayi] - in honor of Professor F.S. Szalay, a leading expert on mammalian skeletal evolution.).

"This mammal could be the great grand aunt or uncle, or it could be the great grandparent of all marsupial mammals," said Dr. Zhe-Xi Luo, curator of Vertebrate Paleontology at Carnegie Museum of Natural History and the lead author of the paper.

Modern marsupials and their extinct relatives make up an important mammalian lineage, known as metatherians, consisting of mammals that are more closely related to modern marsupial mammals (such as opossum, kangaroos and koala) than to placentals (such as humans, rodents and whales). Modern marsupials are a significant part of the larger metatherian mammal group, and are the descendants of the extinct metatherians that lived during the age of dinosaurs, known as the Mesozoic.

With over 270 species, marsupials are the second most diverse mammal group (after placentals with over 4300 species). Marsupials and placentals are both therians mammals characterized by live-birth fetuses, yet they have different reproductive strategies.

Placentals produce better-developed fetuses after longer gestation. In contrast, marsupials give birth the less mature fetuses, and then nurse them for longer periods of time, often in the mother's "marsupial" pouch.

Today marsupials are present mostly in Australia, New Zealand and New Guinea, and in South America. One species, the Virginia opossum, is present in North America. However, in the age of dinosaurs, fossil relatives of marsupials evolved in Asia and North America, before marsupials spread to the rest of the world after the dinosaur extinction.

Prior to the discovery of Sinodelphys, the previously earliest metatherian fossils were some isolated teeth from the 110 million year old sediments of North America. The oldest jaw fragments of metatherians were from deposits of Uzbekistan 90 million years in age. The previously oldest skeletal fossil is from Mongolia and is 75 million years in age.

"The newly discovered Sinodelphys extends the duration for the marsupial lineage by 15 million years, and the earliest record of metatherian skeleton by 50 million years," said Dr. Luo. "This new fossil provided precious, new information about the skeletal anatomy, function, and habits of the earliest metatherians, and sheds light on the evolution of all marsupial mammals."

"The earliest fossils of metatherians are extremely important for scientific studies on the origins of all marsupial mammals, said Dr. John Wible, curator of Mammals at Carnegie Museum of Natural History and a co-author of the paper. "Because marsupials and placentals are close to each other and they dominated the world after the extinction of dinosaurs, the earliest metatherian fossils are also relevant for the understanding the divergence of marsupials and placentals, an important event in the history of vertebrate life."

A nearly complete skeleton of Sinodelphys, preserved on a shale slab, was found in the Mesozoic Yixian Formation in western Liaoning Province of China. The fossil is estimated to be 125 million years in geological age. Around the skeleton are well-preserved impressions of fur and some carbonized soft-tissues.

The mouse-sized animal was about 15 cm (about 6 inches) long and weighed 25 to 30 grams (one ounce). Marsupial-like features can be found in the wrist, anklebones, and in the anterior teeth. The dental features indicate that Sinodelphys ate insects and worms, much like modern-day small mammals.

As with modern tree-dwelling animals, Sinodelphys' shoulder, limbs and feet suggest that it was quite capable of climbing. It was adapted to climbing lower branches of trees and bushes. It lived in woods or shrubs on the lakeshore or riverbank and scurried on uneven surfaces on the ground.

Co-existing with Sinodelphys were the feathered theropod dinosaurs and giant sauropods. There were also pterosaurs, primitive birds, amphibians, reptiles, fish, insects, and diverse plants. Sinodelphys was one of several mammals in the Yixian biota, including: the earliest-known placental-relative Eomaia, the symmetrodonts Zhangheotherium and Maotherium, the eutriconodonts Jeholodens and Repenomamus, and the multituberculate Sinobaatar.

"Interestingly, the more primitive mammals of the Yixian feathered dinosaur fauna were adapted to terrestrial or ground dwelling living," said Dr. Wible. "But only the derived eutherian Eomaia and metatherian Sinodelphys were scansorial or climbing mammals. This suggests that scansorial adaptations were important in the earliest divergence of the modern marsupials and placentals."

A collaborative team of Chinese and American scientists accomplished the discovery and research on Sinodelphys. The Chinese research team was led by Dr. Qiang Ji of Nanjing University and the Chinese Academy of Geological Sciences. The American research team includes Dr. Zhe-Xi Luo and Dr. John Wible of the Carnegie Museum of Natural History.

This research was supported by funding from the Ministry of Land Resources and Ministry of Science and Technology of People's Republic of China (to Prof. Q. Ji), the National Science Foundation of USA (to Z.-X. Luo and J. R. Wible), the National Geographic Society (to Z.-X. Luo), and the funding from Carnegie Museum.