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Earliest Known Ancestor of Placental Mammals Discovered

By John Roach
for National Geographic News
April 24, 2002
 
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 grandparent—albeit 125 million years removed—to 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."

Mammalian Evolution

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 mammal—a 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?

Before the discovery of Eomaia, fossils of isolated teeth indicated the oldest eutherians were 110 million years old. The earliest eutherian fossils with skeletons, however, were 80 million years old.

Recent molecular analyses of mammalian DNA conducted by biologists such as Mark Springer at the University of California at Riverside suggest that the split took place more than 100 million years ago. Until now, however, there was no definitive skeletal fossil evidence to substantiate these studies.

"Our molecular results suggest that living placental mammals have a common ancestor about 105 million years ago," said Springer. "[Other] molecular data also suggest a much earlier split between living placentals and living marsupials—about 175 million years ago. If we evaluate the new fossil in this framework, it is entirely expected that we should find stem placentals that are this old, and older."

Some scientists, such as Weil, speculate that the 175-million-year time frame for the split may be an overestimate. But the discovery of Eomaia, along with four other recently discovered mammalian fossils in the Yixian Formation of the Liaoning Province, indicates that indeed there was great mammalian diversity 125 million years ago.

"Diversity can be used in a variety of senses," said John Wible, a colleague of Luo's at the Carnegie Museum of Natural History and a co-author of the study.

"It can mean species richness, and so the discovery of new forms automatically increases species richness," said Wible. "It can mean morphological diversity, and Eomaia has unique features in its dentition and skeleton. It can mean ecological diversity, and Eomaia was found in lake deposits, unlike most other Cretaceous placentals."

The Life of Eomaia

The feet of Eomaia indicate that the tiny creature was adapted to climbing trees and bushes, where it probably fed on insects much like modern-day shrews, said Luo. Like other small vertebrates, Eomaia was probably nocturnal and preyed upon by carnivorous dinosaurs, such as Sinosauropteryx, a feathered dinosaur.

"One specimen of Sinosauropteryx was found with two mammal jaws and lizard skeletal parts in the pelvis," said Luo. "Obviously, Sinosauropteryx ate and digested small mammals. But this is nothing special for Eomaia. All small vertebrates of the similar size range of Eomaia would be prey of larger predators, most of which were dinosaurs then."

Weil, who wrote an accompanying perspective on this discovery in the April 25 issue of Nature, said the diversity of fossils recently discovered from the Yixian Formation gives researchers a snapshot of what the world was like when eutherian animals were just one mammalian lineage among many.

"So many Mesozoic mammals are known only from teeth," she said. "In this case, every mammal described is known from a skull or skeleton, and it will allow us to learn a lot about the ecosystem in which our earliest ancestors lived and the niches that the mammals that were alive then filled."
 

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