Scientists have found bones of the first land mammal ever discovered in Antarctica, the dramatic and long-sought animal evidence to confirm the theory of continental drift.

Calling the discovery "one of the most significant scientific discoveries in recent years," the National Science Foundation, which sponsors all U.S. Antarctic work, announced the finds yesterday.

One of the long-standing puzzles of evolution has been the connection between the isolated, but obviously related, colonies of marsupial mammals in South America and Australia. If these mammals originated in the Americas, as seems likely, how did they get all the way to Australia?

The discovery of the three jaw pieces and several tooth fragments of a squirrel-like animal that lived in Antarctica 50 million years ago supplies an important piece of evidence for two theories, that the marsupials migrated to Australia across Antarctica when that continent was warm and habitable and that the continents of South America, Antarctica and Australia were once placed differently on the globe and were joined.

Scientists have sought evidence of early mammals in Antarctica for more than 60 years. In a 1931 book, geologist Lawrence M. Gould, who was second-in-command of Adm. Richard Byrd's second polar expedition, wrote, "I had rather go back to Antarctica and find a fossil marsupial than three gold mines."

The discovery was made on Seymour Island at the northeast tip of Antarctica March 7 by Sankar Chatterjee of Texas Tech University, one of a team of researchers led by polar specialist William Zinsmeister of Ohio State University, and including Michael P. Woodburne of the University of California at Riverside and Rosemary Askin of the Colorado School of Mines.

The marsupial family of mammals, which includes such creatures as koala bears, opossums, wombats, bandicoots and kangaroos, is an odd group of creatures distinguished chiefly by the fact that their babies are not carried in a womb, but in an external pouch.

The animals are now found chiefly in Australia and South America; none has ever populated Africa, Europe or Asia.

"The confirmed presence of land mammals in Antarctica clearly shows that Antarctica and South America were attached about 65 million years ago," Zinsmeister said. It was during that period that marsupials which originated in South America would have moved to what were the dense forests of Antarctica.

The animal fragments found in Antarctica were given preliminary dates of 50 million years. The marsupials in Australia date back only about 25 million years, which fits the time sequence for the theory of marsupial passage from the Americas to Australia.

The theory of continental drift, which has become the dominant view of the earth's geology recently, holds that all the continents were at one time bunched in a single mass, which scientists call Pangaea.

The continents, it is now believed, lie on movable plates of the earth's crust. Over the past 150 million years the plates have broken up, drifted apart and in some cases collided again to form the continents as they are today.

Marine animals, reptiles and a variety of plants have been found in Antarctica before, showing that the land was once habitable. But until now no one has found evidence of the Antarctic marsupial which has figured in the theories of evolution and continental drift.

The particular variety of marsupial found is of an extinct family called polydolopida, an animal that lived in forests and ate leaves and berries.

Other finds in the polar expedition which started Feb. 17 and ended March 9 include many fossil bones, some of which were those of giant penguin which stood 6 feet tall, fossilized shark teeth and the first discovery in Antarctica of a Mosasaur, a marine lizard that lived 70 million years ago.