The Japan Times, December 4, 1998

Mongolian fossils give clues to evolution of marsupials

LONDON (Reuters) Two 80-million-year-old fossils found in Mongolia's Gobi Desert have given scientists vital clues to the evolution of pouched mammals such as kangaroos and wombats.
The fossils allowed the ancient opossumlike Deltatheridium to be classified firmly as a marsupial, and supported the theory that the mammal group, now most common in Australia, originated in Asia.
"This gives us a full set of clues for the origins of one of the three major groups of living mammals," Michael Novacek, the curator of vertebrate paleontology at the Museum of Natural History in New York, said Wednesday.
"This is probably the best root of the marsupial branch of the tree that we have found so far."
The research by Novacek and colleagues from the museum and the Mongolian Academy of Science is published in the latest edition of the science journal Nature.
Marsupials, which rear their young in pouches, are one of three types of mammals living today. The others are monotremes - egg-laying mammals including animals such as the duck-billed platypus and the anteater - and placentals such as humans.
Until the discovery of the fossils, scientists were not sure if Deltatheridium was a marsupial or placental.
One of the fossils is of a young animal that died before its adult teeth had completely emerged, revealing the sequence in which they grew. Marsupials do not replace as many teeth as other mammals.
"Only the last three molars are replaced. It's very specialized and that is the first clear evidence we have of this marsupiallike replacement in these very weird and ancient creatures," said Novacek.
The new specimens also support the theory that marsupials may have evolved in Asia and then migrated across North America and down to South America and subsequently to Australia.
The second fossil is of an adult animal with a nearly complete skull, jaw and arm bones.