At the edge of a sprawling grassland, a pair of hoofed grazers resembling horses, an antelopelike notoungulate and a ground sloth feed quietly, oblivious to their impending doom. Equally unaware are the chinchilla and the tiny, mouselike marsupial nibbling seeds nearby. Suddenly, one of the jagged, snow-covered volcanoes on the horizon explodes catastrophically, sending a flood of muddy ash down its steep slopes. Soon after, this roiling slurry bursts across the flatter lowlands, entombing the unsuspecting animals in its path.
As devastating as this volcanic torrent was for the creatures it buried, it would become a boon for paleontology. Tens of millions of years after the mammals' untimely deaths, the exhuming forces of mountain building and subsequent erosion exposed remnants of their fossilized skeletons to the light of day high in the Andes Mountains of central Chile. Our team discovered the first of these bones in 1988 while searching for dinosaur remains in an alpine valley of the Tinguiririca River, near the border with Argentina. The initial finding of mammal bones proved so fruitful that we have returned to the region nearly every year since. So far we have uncovered more than 1,500 fossils of ancient mammals from dozens of sites in the central Chilean Andes.
Painstaking laboratory analysis of our growing collection has yielded major revelations about the history of South America's ancient mammals. To our astonishment, the Chilean fossils range from 40 million to 10 million years old--much younger than anything we expected to find there. Indeed, many of the specimens represent the only mammal remains from segments of that time interval found anywhere in South America. Some of these unique fossils illuminate a previously opaque period in the history of the continent's native mammal lineages; others help to resolve long-standing debates about the origins of key immigrant groups. Together they have revised understanding of when certain ecosystems--and the mountains themselves--appeared in this part of the world.
Most of what scientists know about South America's ancient mammals is based on clues unearthed in the continent's far southern reaches, mainly Patagonia. Those regions have abundant outcrops of typical fossil-bearing rocks--shale, sandstone and other hardened sediments from rivers and their floodplains. Before our first visit to Chile, researchers had not searched systematically for land animal fossils in that country's mountainous areas, because most rocks there are volcanic. (The standard assumption is that lava and other erupted materials are too hot and violent to preserve organic remains.)
We decided to take a chance that the Tinguiririca Valley might harbor fossils when we learned of a report about dinosaur footprints. The rocks were the right age--geologists then assumed that most rocks along the main spine of the Chilean Andes dated back at least 65 million to 100 million years, to the latter part of the Mesozoic era, when dinosaurs reigned supreme. We knew that any sediments preserving footprints might also contain bony remnants of the track makers. If we were extremely lucky and kept our eyes close to the ground, we might even find a fossil of one of the dinosaurs' tiny mammalian contemporaries, which were no bigger than shrews.
On the last day of a one-week reconnaissance trip in 1988, our team of four split up to prospect the precipitous slopes flanking each side of the Tinguiririca River. Almost immediately, the pair working north of the river reached the layer of ancient sediments that bore the dinosaur tracks, then continued up the valley in search of more potential fossil-bearing deposits. To their dismay, however, the only fossils they recovered were from fish, ammonites and other ocean-dwelling creatures--no reptiles or mammals. Meanwhile the team members working on the south side of the river were having a similarly frustrating day. Late in the afternoon, though, their spirits soared when they spied a few fossilized scraps of bone and teeth eroding out of a large patch of reddish-brown volcanic sediments nearly 1,000 meters above the valley floor. A closer look revealed that the fossils were land-dwelling vertebrates about the size of a small horse.