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 marsupialsabout 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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