History of mammals: new clues from Antarctica

Some 70 million years ago the forbidding continent of Antarctica was a far different place. It thronged with prehistoric life, including the predecessors of many modern species. It may also have played a pivotal role in the development of many early mammals, including those still found in Australia.

This picture of the now ice-shrouded continent has been given considerable new support by fossil finds from the Palmer Peninsula. These offer the first direct evidence of what happened to life in Antarctica over the last 100 million years as its climate gradually changed from semi-tropical to polar.

US paleontologists on a multidisciplinary expedition sponsored by the National Science Foundation say they have found what appear to be the first fossil remains of an Antarctic land mammal, as well as other fossil evidence.

After three weeks of exhaustive searching - battling 40-mile-an-hour winds and thick mud - the scientists had almost given up finding clear evidence of an Antarctic mammal. On their next to last day, however, they found what they had been looking so hard for.

''The fossil is that of a small berry-eating marsupial similar to those found in South America,'' reports William Zimsmeister of Ohio State University. Discovered were some bones of animals about the size of a dog, which detailed analysis may also confirm as mammalian, he adds.

The small marsupial dates to 40 million to 45 million years ago and is the first direct evidence to support the theory that ancestors of Australia's strange coterie of mammals like the koala bear and the platypus arrYved there by way of Antarctica. Previously it was thought they migrated from North America, where mammals appear to have originated, by way of Asia. But increased understanding of land configurations in this period have made an Antarctic route seem more likely.

From their finds, the scientists hope to piece together a detailed picture of life in this area at the dawn of the Cenozoic era, which began 65 million years ago and extends to the present. It is characterized by the appearance and development of the mammals, including man.

''This was a critical interval in earth's history,'' explains Dr. Zimsmeister. He and Michael Woodburne of the University of California at Riverside made the recent finds. ''What happened in Antarctica then greatly influenced [the type of life] we see today,'' he elaborates.

Seventy million years ago Antarctic shores and seas ''abounded'' with life, Dr. Zimsmeister says. The climate was probably semi-tropical. Giant reptiles like the plesiosaur paddled in its waters, accompanied by bony fishes two to three feet long. The land was mantled in dense forest with trees hundreds of feet in height. Not too much is known about the land animals that dwelt there, but the recent expedition did find what they believe to be the fossil remains of a number of different bird and lizard species.

At that time Antarctica was part of a supercontinent, dubbed by scientists Gondwanaland. It included South America, Africa, peninsular India, Australia, and Antarctica. But inexorable geologic tides were breaking up this giant landmass. About 60 million years ago Australia broke off and began moving north. Twenty million years later, South America separated and began moving in a westerly direction. Antarctica stayed about where it was, but these major geographic changes precipitated severe environmental changes. Its plants and animals were forced to adapt.

By the Eocene epoch, 54 million years ago, the Antarctic had already begun to experience these changes. The land was still covered by dense forest, probably like that which covers Tasmania today. The oceans still thrived with life, but the reptiles had been replaced by six-foot penguins, sharks, and whales.

''From there everything went downhill,'' Dr. Zimsmeister comments. Some 28 million years later the forests and land animals had disappeared. The continent was covered with ice much as it is today.

These major changes make Antarctica an ideal place to study how various life forms respond to environmental change, the scientist argues. ''It's a problem we may have to face sometime in the future.''

While the area may be ideal from a scientific point of view, its harsh environment is far from perfect for the discovery of fossils. But Dr. Zimsmeister believes the expedition has found a site that could prove to be a ''Rosetta stone'' of mammalian evolution in the South Pacific. This is an odd island in the Palmer Peninsula. ''It looks like the Badlands [in South Dakota] . . . the only way you know you are in the Antarctic is by the occasional penguin, '' the scientist says. ''But it is the most [fossil-rich] place I've ever been, '' he exclaims. And its rocky surface dates from 40 million to 70 million years old, he says.