Oldest marsupial found?

Oldest marsupial fossil found?

When one thinks of marsupials today, images of perky kangaroos and slow-moving koalas beckoning tourists to Australia come most readily to mind. But because Australia's fossil record is rather sparse, paleontologists looking for traces of ancient marsupials have had more success hunting in the Americas. Now scientists have discovered yet another such fossil in North America, which may be the oldest marsupial remain in the world.

Last August, Jeffrey Eaton, a graduate student at the University of Colorado in Boulder, found a marsupial lower jaw and three molars embedded in a southern Utah rock formation that dates from the second half of the Early Cretaceous epoch, about 100 million years (Myr) ago. Previously, the oldest undisputed marsupial fossil was a Canadian specimen that is about 20 Myr younger. The oldest reported marsupials in South America lived about 65 to 70 Myr ago, and the oldest marsupial fossils in Australia are less than 30 Myr old. One researcher has argued that Texas fossils dated several Myr older than the Utah jaw are marsupial remains, but this claim is not widely accepted, according to a number of scientists.

Richard Cifelli, a curator at the Museum of Northern Arizona in Flagstaff, who is studying the Utah jaw, thinks the recent find is particularly exciting because Early Cretaceous mammal fossils-- especially pieces larger than teeth, which in such mammals can be microscopic in size--are extremely rare. Moreover, Cifelli and Eaton say the teeth in the Utah jaw are surprisingly advanced, resembling those of a family of extinct marsupials that lived 30 Myr later.

By showing that marsupials had arisen and specialized by 100 Myr ago in North America, the jaw also gives clues to the origin of these animals. According to Eaton, paleontologists have long considered South America as the place where marsupials probably originated--in part because they thrived there for so long and also because Australia was once linked to South America by Antarctica. The Utah jaw, say the researchers, does not preclude a South American origin, but it makes North America the more likely candidate.

Eaton and Cifelli are working in isolated and largely unexplored parts of Utah, which they believe are rich in mammal and dinosaur remains from the Cretaceous. Says Jason Lillegraven at the University of Wyoming in Laramie: "I rank this project as one of the five most important research efforts going on in mammalian paleontology today.'

Photo: Marsupial jaw and molars.