How can there be marine fossils on the top of mountains?
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Scientists have found fossils of whales and other marine animals in mountain sediments in the Andes, indicating that the South American mountain chain rose very rapidly from the sea.
The rare assemblage of fossils, recovered on an expedition by the American Museum of Natural History to a remote plateau in southern Chile, is expected not only to illuminate an obscure epoch of animal evolution but also to document the rise of the Andes mountains in the past 15 million years.
Among the fossils the scientists reported bringing back were the bones of whales and other marine animals found at altitudes of more than 5,000 feet. When these animals died from 15 million to 20 million years ago, their carcasses settled to the ocean floor and were embedded in submarine sediments. But since then, the violent upthrusting of the Andean chain has carried the sediments to the tops of mountains. In geological terms, the time the fossils took to rise from ocean floor to mountain top was relatively brief.
According to the leader of the expedition, Dr. Michael J. Novacek, the chairman of the museum's paleontology department, the presence of interesting fossils on the plateau was detected by an amateur Chilean paleontologist who had been the mayor of a local town.
The place is truly a lost plateau reminiscent of the settings of adventure novels, a strange and wild place that had entirely escaped the attention of scientists. There are no roads into the area, and the fierce mountain winds are too dangerous for helicopter operations.
The plateau, some 850 miles south of Santiago near the border between Chile and Argentina, lies just to the south of Lake General Carrera. The lake is the second largest lake in South America after Lake Titicaca, which straddles the border between Bolivia and Peru.
Assemblages comparable to this are virtually unknown in the Andes, he said, since geological upthrusting generally destroys fossil beds. 'Remarkably Intact' Fossils
Nearly all of the fossils were embedded in surface rock and easy to pick up, he said. ''That was another great piece of luck, since we couldn't have brought in excavating equipment on horseback,'' he said. ''Best of all, despite weathering, many of the smallest fossils were remarkably intact and will be relatively easy to study.''
The collection represents both sea and land animals, and through the 1,000-foot thickness of the main butte the group explored, the transition from oceanic to terrestrial environments was preserved in a smooth gradient.
For example,'' Dr. Novacek said, ''we found the oyster beds and sand dollars just beneath the lowest sediments containing land animals. At that point the water was shallow and receding rapidly - a time of transition from sea to land, as the land was thrust up by magma and the movement of tectonic plates.'' In more recent sediments, the group found species related to modern rodents, porcupines, rhinoceroses and camels. Among the many fossil curiosities they came across were ungulates (including a rabbit-like ungulate), marsupials and giant sloths. Clues on Joining of Continents
These animals, lived 12 million years before South and North America were linked by a land bridge, and therefore represent groups that evolved differently from their relatives in North America. By precise dating of the sediments in which their bones were found, he said, geologists will greatly improve knowledge of when and under what circumstances land bridges existed.
There is evidence, he said, that a shallow body of water may have once completely severed southern South America from the rest of the continent. A long bay that once stretched from the Atlantic coast of South America to a point near the plateau that the expedition studied may have been a remnant of such a transcontinental channel.
''Among the best indicators of geologic change,'' are rodents and other small animals. They ranged over much smaller distances than large animals, and changes in their populations often reflect local geological changes.''
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