The fossil of a tiny creature found in Northeast China is helping scientists determine when mammals split into different groups: those with babies that develop inside their mothers and those that raise their offspring in pouches.
The two groups make up more than 99 percent of all mammals today, and the new fossil evidence indicates the separation began in Asia about 125 million years ago.
The newly found ancient animal, named Sinodelphys szalayi, is the earliest known marsupial, meaning an animal with a pouch. It was chipmunk-sized, about 6 inches long and weighed about an ounce, according to Zhe-Xi Luo of the Carnegie Museum of Natural History in Pittsburgh.
Its skeleton was found in 2000 in a region where researchers had previously found Eomaia, a fossil believed to be among the earliest known placental mammals, of about the same age. The discovery is reported in Friday's issue of the journal Science.
The finding of the two fossils indicates an approximate date for the split between the placental and pouch mammals, Luo said in a telephone interview.
"All this was happening while dinosaurs dominated the world. Mammals were able to explore a different niche and able to hang on," Luo said. "Otherwise there wouldn't be any cute koala bears to look at."
Luo noted that Sinodelphys had hands and feet with fingers and strong wrists well adapted to climbing, indicating it could retreat to the trees to avoid hunting dinosaurs.
The discovery also adds support for the 19th century theory of Thomas Henry Huxley that the first marsupials lived in trees.
Luo said evidence indicates that marsupials developed in Asia, spread to North America and then moved southward to South America and Australia. Today marsupials are most common in Australia and South America, with only fossil remains in the northern hemisphere. Opossums, the only marsupial in the north, are a recent immigrant.
"It's an interesting paradox," Luo said. "If you look across the world, South America and Australia, you find the present-day marsupials in great abundance and diversity. But those are not the original points where they evolved."
The find provides unprecedented information on the biology of early marsupials, supporting the theory they lived in trees, Richard L. Cifelli and Brian M. Davis of the Oklahoma Museum of Natural History comment in an accompanying article.
Further evidence is needed, however, to indicate whether that was important in their diverging from placental mammals, they added.
The discovery is important because it helps support evidence for the dates of early mammals developed in other research, said Davis and Cifelli.
Today more than 99 percent of mammals _ animals that provide milk for their young _ are either marsupials like kangaroos and bandicoots, or placental mammals like humans, cats, dogs and others. The remaining small group of mammals, which lay eggs, are called monotremes _ the platypus and echidna.
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