The discovery has changed scientists' theories about how and when marsupials evolved

December 2, 1998
Web posted at: 7:36 p.m. EST (0036 GMT)

LONDON (CNN) -- The discovery of 80 million-year-old fossils in Mongolia is shedding new light on the evolution and migration of marsupials, mammals who develop their young in a pouch.

While scientists have long suspected that marsupials date back to the time of the dinosaurs, few fossils have been found to show how they evolved into today's marsupials, a group that includes opossums, wombats and kangaroos.

Researchers from the University of Louisville, New York's Museum of Natural History and the Mongolian Academy of Science found the remains of an ancient opossum-like creature called Deltatheridium in Ukhaa Tolgod, Mongolia.

Their analysis, to be published this week in the British journal Nature, shows for the first time that Deltatheridium was indeed a marsupial and roamed the Earth during the age of dinosaurs.

Fossils of the extinct deltatheroidans were found in Mongolia

"This gives us a full set of clues for the origins of one of the three major groups of living mammals," said Michael Novacek, one of the researchers. "This is probably the best root of the marsupial branch of the tree that we have found so far."

In addition to marsupials, the three types of mammals living today include monotremes, which reproduce by laying eggs, and placentals, in which offspring are grown internally. Examples of a monotreme would be the platypus and the anteater; humans are placentals.

Until the discovery in Mongolia, scientists weren't sure whether Deltatheridium was a marsupial or a placental. But the fossils show that the pattern of growing teeth in Deltatheridium was akin to that of marsupials.

The discovery casts doubt on one widely held theory: that some of today's marsupials might have existed during the age of the dinosaurs.

But finding the remains of an ancient marsupial ancestor in Asia supports the theory that this branch of the mammal family may have first evolved in Asia, migrated across North America and down to South America and subsequently to Australia, which is home to a number of marsupial species today.

Reuters contributed to this report.