Scientists have found the oldest known member of the marsupial group of mammals whose present-day relatives include kangaroos, koalas and opossums.
The mouse-like animal lived more than 125 million years ago. A fossilised skeleton of the ancient marsupial was found in north-eastern China. The discovery shows that mammals with pouches - which are used to suckle their young - evolved some 50 million years earlier than previously thought, said Zhe-Xi Luo, curator of vertebrate palaeontology at the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania.
"This mammal could be the great grand-aunt or uncle, or it could be the great grandparent of all marsupial mammals," Dr Luo said.
Mammals are divided into those that gestate their young for long periods using a placenta for nourishment - called placentals - and those that give birth to immature offspring, which are then fed milk via a nipple in an external pouch, the marsupials.
Marsupials are the second most diverse group of mammals with more than 270 known species. Placental mammals, with more than 4,300 species, are considered more successful because of their ability to out-compete marsupial rivals when the two live side by side. The nearly fossilised skeleton - published in the journal Science - is of an animal weighing about 25 to 30 grams (1oz) and about 15cm (6ins) long.
Its wrist, ankle bones and teeth all show clear marsupial features, Dr Luo said. Its skeleton was preserved on a slab of shale found in the fossil beds of western Liaoning Province, China. It would have lived alongside ancient reptiles and amphibians when feathered dinosaurs, flying pterosaurs and giant plant-eating sauropods dominated the landscape.
Called Sinodelphys szalayi, the furry, insect-eating creature was probably nocturnal, climbed trees, and lived in woods or shrubs on a lakeshore or riverbank, scurrying occasionally on the ground, the scientists said. "This new fossil provided precious, new information about the skeletal anatomy, function and habits of the earliest metatherians [primitive marsupials], and sheds light on the evolution of all marsupial mammals," Dr Luo said.
John Wible, curator of mammals at the Carnegie Museum, said: "Because marsupials and placentals are close to each other and they dominated the world after the extinction of the dinosaurs, the earliest metatherian fossils are also relevant for understanding the divergence of marsupials and placentals, an important event in the history of vetebrate life."
The discovery was a collaboration between Nanjing University, the Chinese Academy of Geological Sciences and the Carnegie Museum.Reuse content