A team of Chinese and American scientists say they have found a 125-million-year-old fossil of an animal that is the most primitive known relative of today's higher mammals, including humans and primates.
The remains of the creature, Eomaia scansoria, push back the fossil records of so-called placental mammals by millions of years and provide a wealth of information about them.
Mammals that nourish their young in the womb through an organ called the placenta account for the vast majority of all mammals, with a few notable exceptions such as marsupials.
"This mammal could be our great, great aunt or uncle, or it could be our great-grandparent 125 million years removed," said Dr. Zhe-xi Luo of the Carnegie Museum of Natural History in Pittsburgh.
"Across a wide range of mammals we all share one common ancestry. We are all placental mammals. With this new fossil we can trace the root of all the placental group," he added in an interview.
Inherited, newly evolved features to be distinguished
The tiny creature, which was no bigger than a large mouse, scurried on the ground at the feet of the dinosaurs and may have made a tasty treat for some of its monstrous contemporaries.
Its detailed features -- teeth, foot bones and fur -- suggest it lived in low branches and bushes, was adapted for climbing and fed on insects.
"We have extended the quality record to the earliest time interval of placental evolution," Luo explained.
The fossil will allow scientists to determine which features placental mammals have inherited from their earliest ancestors and which are newly evolved characteristics of the group.
"In order to distinguish those two possibilities for any anatomical features, we have to trace back to this earlier fossil record. That is why it is so exciting," said Luo.
Before the discovery of the fossil, which is reported in the science journal Nature, the earliest record of a placental mammal was a few isolated 115-million-year-old teeth.
The new find was discovered in a quarry in the Liaoning Province of China, an area where remains of feathered dinosaurs and very primitive birds have also been found.
( April 25, 2002)
