The skeleton, complete with a dense pelt, surpasses by some 15 million years the previously oldest known representative of this lineage.
The remains, entombed in slabs of prehistoric rock, provide a treasure trove of details about early mammalian evolution and ecology, said the study authors from the Carnegie Museum of Natural History in Pittsburgh, Pa., and the Chinese Academy of Geological Sciences and Geoscience University of China in Bejing. Their report will be published April 25 in the British journal Nature.
Though the skull of the 5-inch critter is squashed flat, its teeth, tiny foot bones, cartilages and even fur remain nearly intact, providing a rare window into the life and times of eons ago.
Prior to the discovery in 2002 by local farmers prospecting for fossils in an area of Liaoning Province, famed for its rich yield of remnants of ancient lifeforms, including feathered dinosaurs, the earliest record of a placental mammal was represented by three 110-million-year-old teeth unearthed in Siberia.
"Our new study shows that in the Cretaceous (a period from 140 million to 65 million years ago marked by the rise and fall of the dinosaurs), there was a far greater burst of diversity of extinct relatives of placentals than anyone had previously realized," said study co-author John Wible, associate curator of mammals at Carnegie.
"We found the earliest ancestor -- perhaps a great-great uncle or aunt or perhaps a great grand-parent (albeit 125 million years removed) -- to all placental mammals," said study co-author Zhe-Xi Luo, associate curator of paleontology at Carnegie.
"The great majority of the mammals that we humans are familiar with are placentals: apes, monkeys, flying lemurs, rats, squirrels, rabbits, elephants, manatees, armadillos, tree-sloths, bats, horses, rhinoceros, lions, tigers, dogs, pandas, pangolins, pigs, sheep, cows, camels, hippopotamus, dolphins and whales," he told United Press International.
"Because so many animals of economic and ecological importance are placentals, because so many animals that humans love are placentals and because humans and other primates are placentals, the earliest fossil records of eutherian mammals are extremely important for understanding the origins and early evolution of all placental mammals."
The fossil of Eomaia -- Greek for "ancient mother" -- scansoria -- Latin for "climber" -- is not only the earliest eutherian mammal known, but also one of the best preserved of all mammals from the Mesozoic Era, some 230 million to 65 million years ago, he said.
The discovery in the famed feathered dinosaur quarry in northeastern China, called the Yixian Formation, shakes up the family tree of modern eutherian mammals.
This mammalian group -- the most diverse in the world -- is characterized by the placenta, an important evolutionary innovation that allows a much longer gestation for a more mature fetus at birth. Living placental mammals, such as humans, are distinct from marsupials, such as kangaroos, and monotremes, such as duckbilled platypuses, in possessing the placenta and certain dental and skeletal features the other two groups lack.
"The beautiful preservation has two important results:" It should resolve the debate about whether Eomaia is truly a eutherian and it should shed unprecedented light on its way of life, said Anne Weil, research associate in biological anthropology and anatomy at Duke University in Durham, N.C., who wrote an accompanying commentary.
"I suspect (a complete skeleton) would decide everyone immediately about that animal," Weil told UPI. "But most early mammals are known from only their teeth or from jaw fragments."
The well-preserved remains also allow scientists to compare the entire skeleton to that of living mammals, and make an educated guess about the animal's lifestyle, she added.
"In many respects, the fossil is exactly in line with scientific predictions -- we thought that there should be eutherian mammals that old, we thought they should have hair, we thought they would probably be pretty small in size," she said.
The animal's elongated fingers suggest it was an adept climber, scurrying up trees and bushes and around hidden crevices to avoid getting trampled -- and perhaps devoured -- by dinosaurs, scientists speculated.
Larger than a mouse, smaller than a squirrel, the animal measured an estimated 14 centimeters (about 5 inches) nose to tail-end and weighed about 20 to 25 grams. Its dentition indicates it had an appetite for insects.
"You can imagine that this small mammal, insectivorous and an agile climber, lived on a lush shore of a fresh-water lake that was teeming with all walks of life -- insects, plants and vertebrates," Luo told UPI.
"This find is cool by itself. But in the greater context of mammals from the Yixian, it is fascinating! With all the attention to feathered dinosaurs, this amazing mammalian fauna has been scuttling under the radar," Weil told UPI.
"There's a lot more to this fossil than 'the earliest eutherian!' or 'the earliest fur!' It is not an isolated find; here we get to see an early eutherian in the context of a complex, 125-million-year-old ecosystem."
And as such, it gives humans a glimpse into their own past, researchers said.
"A better understanding of our mammalian heritage gives a great meaning to our own existence," Luo told UPI. "On practical terms ... the mammalian family tree is the road map for scientists to decipher the genetic make-ups of all mammals, including human, and for understanding the history behind our biology."