Did Mammals Spread from Asia? Carbon blip gives clue to animals’ Eden

A new dating of Chinese fossils buttresses the idea that an Asian Eden gave rise to at least one of the groups of mammal species that appeared in North America some 55 million years ago.

LOOKING FOR ANCESTORS. Red sedimentary layers in Wyoming mark the first known North American occurrences of several mammal groups, including the hyaenodontids. Science/ P. L. Koch

By analyzing the carbon and magnetic characteristics of rocks in the Lingcha Formation in Hunan, China, scientists have been able to assign fossil-bearing geologic strata to a dramatic period in the history of modern animals, says Gabriel J. Bowen of the University of California, Santa Cruz. At the time, the dinosaurs were long gone, and mammals were taking over. A burst of global warming coincided with the North American and European debut of newfangled mammal groups and departure of archaic ones.

With their more precise Asian dating, scientists are matching Chinese fossils with contemporaneous remains from other continents. In the March 15 Science, Bowen and his colleagues explain their dating method, which provides evidence that a family of predatory mammals called hyaenodontids, now extinct, originated in Asia before they appeared in North America. These dog-size predators filled niches now occupied by wolves and wild cats.

The team dated the Chinese fossils in part by examining the isotopes of carbon in hardened nuggets of calcium carbonate that precipitated in the surrounding soil. As the scientists worked their way through layers of the Lingcha

Formation, the ratio of carbon-13 to carbon-12 plunged dramatically and then rebounded. That sharp dip marks those strata as coming from an 80,000-year stretch when the Paleocene epoch gave way to the Eocene. Other scientists have proposed that the dip in the carbon-isotope ratio came from a great burp of methane released from methane hydrates–where carbon-12 is abundant–melting on the ocean bottom.

Researchers also found evidence that Earth’s magnetic polarity at the time the strata were deposited was the reverse of its polarity today. That bolsters the claim that these strata chronicle the Paleocene-Eocene transition because Earth’s magnetic polarity then was reversed. In Europe and North America, the Eocene witnessed the appearance of ancestors of today’s primates and ungulates, which include horses and cattle.

Using a statistical technique, Bowen’s group extrapolated their chronology to several dozen fossil-rich sites in China and Mongolia.

Hyaenodontid fossils are found in pre-transition strata in China, but only in more recently deposited strata in North America. This suggests that hyaenodontids migrated from Asia. The results are less definitive for primates and ungulates.

Nevertheless, Christopher Beard of the Carnegie Museum of Natural History in Pittsburgh, and a major advocate of the out-of-Asia movement for modern mammals, greets the new research enthusiastically. Says Beard, it “strengthens Asia’s claim as the birthplace of numerous groups of mammals.”

Another paleontologist who studies early mammals is less convinced. “The idea of everything coming from Asia is greatly oversimplified,” says Philip D. Gingerich of the University of Michigan in Ann Arbor. “Now, we have this very important time marker on all three continents.”

But for the question of whether many mammals spread from an Asian Eden, Gingerich says, “this is not the answer.”

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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