Fire, Ice & Fossils

Natural History, June, 1999 by Andre Wyss, John Flynn, Reynaldo Charrier

Born of lava and shifting earth, the glacier-topped Andes hold clues to a continent's past.

Ever since Charles Darwin crossed the Andes by mule in 1835 and then published his pioneering geological study of the region, the mountain chain has been one of the most intensively researched in the world. Even so, geologists and paleontologists continue to encounter major surprises there. Ten years ago, our international team of investigators discovered the first of a series of new fossil mammal sites in the rugged terrain of the Chilean Andes. To date we have collected specimens representing dozens of mammal species, most of them new to science. These fossils provide clues vital to deciphering the geological history of this vast range and fall a key gap in the fossil record, a 20-million-year period during which South America, separated from all other landmasses, was a great island continent.

Two major, and related, forces that shaped South America--and the evolutionary history of its animals--are plate tectonics and volcanism. The Andes are part of the Ring of Fire, an immense zone of seismic and volcanic activity that borders much of the Pacific Ocean. For millions of years, and continuing today, a giant slab of ocean crust has been sliding deep into the earth's interior, below the western margin of South America. As this slab gradually melts, it forms huge pods of magma that slowly ooze and melt their way upward, some reaching the surface as volcanoes. Buckling of the continental margin has lifted the Andes into existence, and subsequent volcanic eruptions, up to the present day, have added a six-mile-thick layer of rock and debris, accounting for the region's spectacular scenery. Often these eruptions generated catastrophic mudflows that inundated valleys and buried wildlife, the remains of which we have been excavating as fossils.

The Andes were still young, and considerably less lofty, when the landmasses that made up the southern supercontinent of Gondwana--South America, Africa, India, the Middle East, Antarctica, and Australia--began to separate some 180 million years ago. As the Andes grew, Gondwana continued to break apart, and by some 70 to 80 million years ago, the major dry-land connections between South America and neighboring continents were cut. From then until about 3 million years ago, when the Central American isthmus forged a connection to North America, South America was isolated much as Australia is today. This lengthy separation left South America's ecosystems to evolve with minimal outside influence. The continent's early mammals were every bit as unusual as those of present-day Australia.

We began our fieldwork near the small summer-resort village of Termas del Flaco, which lies along Chile's west-flowing Tinguiririca River and deep within the Andean main range. The existence of a dirt road to the town (providing rare access in this high-altitude region) and a 145-million-year-old dinosaur trackway preserved on a nearby limestone slab first drew our attention to this area. Given the tracks, we reasoned that the bones of dinosaurs or their vertebrate contemporaries might lie buried there as well.

Earth scientists had long thought that the rock formations in the vicinity of Termas del Flaco fell into two age categories: some were roughly 100 million years old or older and others less than about 2 million years old, with none of intermediate age. Because these formations were notoriously difficult to date accurately, and no age-diagnostic fossils had yet been found, dating had been based on interpolation and estimates of the rate at which volcanic sediments were deposited. So, in 1988 our team was completely surprised to find fossils--at first just teeth and pieces of jaws--characteristic of mammals that lived about 35 million years ago. Our colleague Carl Swisher analyzed the argon gas trapped within the mineral crystals (using a new and extremely precise method of radioactive dating) and verified that the fossils, and the rock formation they were found in, were between 37 and 31 million years old. Known as the Abanico Formation (from the Spanish for "fan"), this rock layer runs for hundreds of miles along the central spine of the Andes. Folded and faulted, it reflects the west-to-east, accordion-like compression of the shifting continental crust.

Fossil mammals from this time period, at the juncture of the Eocene and Oligocene epochs, had never before been reported from South America. The Tinguiririca Fauna, as we named these animals, includes marsupials, an early sloth, armadillos, the earliest known South American rodents (see "Out of Africa, Somehow," opposite), and the earliest record of several groups of notoungulates. A large, diverse group of hoofed herbivores endemic to South America, notoungulates ranged from rabbit- to hippo-sized and over time evolved to fill a variety of ecological niches on their home continent. The last notoungulates became extinct less than a million years ago.

The ancient environment inhabited by the notoungulates and other creatures of Tinguiririca was far different from the sparsely vegetated, unrelentingly steep terrain where we search for bones today. We have uncovered fossils at elevations up to 11,000 feet; at higher altitudes, the rocks are too steep to prospect or are covered year-round by snow or glaciers. Our most frequent visitors are condors, which circle overhead for hours, unused to the sight of large bipeds hammering rocks. During our austral summer field seasons, from January to March, we are prepared for any weather, from heat waves to blizzards. Some 35 million years ago, by contrast, the global climate was relatively mild, and what is now the area around Termas del Flaco was probably at a considerably lower elevation, the Andes not yet having been uplifted to their full height. The community of plant eaters lived on the plains and in lushly vegetated valleys surrounding the volcanoes, basins in which occasional catastrophic mudflows (and other, volcanic material) came to rest. Most of these areas were grassy, as indicated by the abundance of animals with very high crowned teeth, a specialization for eating grass, which is abrasive. The notoungulates of Tinguiririca are the earliest known mammals from anywhere in the world to have teeth adapted for grazing.