Fossil May Explain Split Between Placental and Pouch Mammals

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% of mammals, and the new fossil evidence indicates the separation began in Asia about 125 million years ago.

The ancient creature, named Sinodelphys szalayi, is the earliest known marsupial, meaning an animal with a pouch. It was chipmunk-sized, about 6 inches long and weighing about an ounce, reported Zhe-Xi Luo of the Carnegie Museum of Natural History in Pittsburgh in the Dec. 12 issue of Science.

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.

The two findings indicate an approximate date for the split between the placental and pouch mammals, Luo said in a telephone interview.

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 dinosaurs.

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.