for National Geographic News
Researchers today announced the discovery of the earliest known ancestor of the group of mammals that give birth to live young. The finding is based on a well-preserved fossil of a tiny, hairy 125-million-year-old shrewlike species that scurried about in bushes and the low branches of trees.
"We found the earliest ancestor, perhaps a great uncle or aunt, or perhaps a great grandparentalbeit 125 million years removedto all placental mammals," said Zhe-Xi Luo, a paleontologist at the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania. "It is significant because a vast majority of mammals alive today are placentals."
Cows, rats, monkeys, lions, tigers, and pandas are placentals. Dogs, rhinoceroses, tree sloths, horses, and whales are placentals. And, of course, humans are placentals.
The fossil of the animal, named Eomaia scansoria, was found in the fossil-rich region of Liaoning Province in China, which has also produced ancient evidence of feathered dinosaurs and primitive birds. Eomaia, which means "ancient mother" in Greek, was five inches (14 centimeters) long and weighed no more than 0.9 ounces (25 grams).
"It tells us that the placental lineage has a much earlier origin than previously thought," said Luo, co-author of a paper on the discovery in the April 25 issue of Nature.
The finding indicates that the earliest extinct relatives of placentals had a much greater diversity than previously thought, Luo said, and "tells us about the ancestral morphology from which all placentals would have descended."
There are three groups of mammals alive today: placentals, which give birth to young that are nourished in the mother's womb; marsupials, such as kangaroos, which give birth to premature and helpless embryos that climb into the mother's pouch, where they continue to develop; and monotremes, which lay eggs.
Most mammals today are placentals.
Mammalian diversity was not so simple 125 million years ago, said Anne Weil, a research associate in the department of biological anthropology and anatomy at Duke University in Durham, North Carolina.
"First of all, Eomaia is not a placental mammal. It is an early, primitive representative of the lineage that eventually led to placental mammals," said Weil. "Properly, it is called a eutherian mammala mammal that is more closely related to living placentals than to living marsupials."
Fossil evidence dates the earliest mammals to 220 million years ago. One question scientists ask is, when did mammals split up into the lineages that gave rise to the three groups alive today?
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