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.