A fossilized mammal discovered in northeast China is possibly the great-grandmother or aunt of modern placental mammals including human beings,say scientists.
The fossil,dating back more than 130million years ago,has been identified as the world's earliest-known eutherian mammal fossil,according to joint research by Chinese and US scientists.
The new discovery has pushed back the history of placental mammals to 15million years earlier than the previously oldest record represented by isolated teeth found in Siberia.
The study of the fossil by an international team of scientists from the Chinese Academy of Geological Sciences and Carnegie Museum of Natural History was published in the prestigious British scientific journal Nature on Thursday.
The mammal has been named Eomaia scansoria,Greek for "mother of all placental mammals".
Ji Qiang,chief research scientist from the Chinese Academy of Geological Sciences said that the origin of the placental mammals was an important part of the history of vertebrate life.The earliest fossil records of eutherian mammals are extremely important for understanding the origins and early evolution of all placental mammals.
Scientists say placental mammals are the most prominent mammals in the world today.They include rats,squirrels,rabbits,elephants,horses,lions,tigers,pandas,pigs,sheep,dolphins,whales,as well as humans and other primates.
Mammals also include marsupials (such as the kangaroo),which reproduce through a pouch,and the platypus which lays eggs."The fossilized Eomaia is represented by an unusually complete skeleton exquisitely preserved.Around the skeleton are well preserved impressions of mammalian fur and a halo of carbonized soft-tissues,"said Ji.
The skeleton of Eomaia is about 14cm long,with an estimated weight of between 200and 250grams.Its teeth have many features of eutherian mammals.
"These features indicate that Eomaia looked like today's mouse,and was capable of climbing and scurrying on the uneven surface of the ground and had adapted to climbing the lower branches of the trees and bushes,"said Ji.
It inhabited a wooded environment on the lake shore or river bank,and lived on insects.
Zhang Jianping,another scientist participating in the study from the China University of Geosciences,said that Eomaia was covered by the ash after a volcanic eruption and well preserved through years of history.
He said that the earth was dominated by dinosaurs more than 130million years ago,while some primitive birds and mammals began to emerge.
The method of reproduction plays a significant role in the process of biological evolution.Placental mammals were more efficient breeders,said Zhang.
The warm-blooded mammals are more capable of adapting to different environments.That is the reason why the placental mammals survived and prospered after the extinction of dinosaurs,Zhang added.
( eastday.com April 26, 2002)
