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

December 2, 1998
Web posted at: 6:11 p.m. EST (2311 GMT)

LOUISVILLE, Kentucky (CNN) -- New fossils found in Mongolia are helping scientists better understand how marsupials, animals like opossums and kangaroos, evolved.

A team from the University of Louisville described a "spectacular" fossil find in this week's issue of the journal Nature.

Up until now, scientists haven't known much about a group of extinct mammals called deltatheroidans. Except for a few poorly preserved teeth, the animals have been almost entirely missing from the fossil record.

The newly found, 80 million-year-old fossils, discovered in Ukhaa Tolgod, Mongolia, include the remains of several animals, including one that was in the process of losing its baby teeth.

Fossils of the extinct deltatheroidans were found in Mongolia

Guillermo Rougier of the University of Louisville and his colleagues unearthed the fossils and worked to decipher where the animals fit in on the mammalian evolutionary family tree.

Working mostly with teeth and jawbones, the team of paleontologists concluded that the Deltatheridium mammals are closely related to metatherians, which include opossums, kangaroos and koalas.

The discovery also has changed scientists' thinking about how and when marsupials evolved.

Dating of the fossils indicates the deltatheroidans would have been contemporaries of the dinosaurs. And the reworked marsupial family tree shows that, contrary to conventional wisdom, none of the now-living groups of marsupials was present when dinosaurs ruled the Earth.