View a slideshow of the authors' fossil-hunting expedition.
Across thousands of square miles of central Chile, massive, tilted layers of sedimentary rock rise majestically above wide, alpine valleys. This breathtaking setting has made fossil hunting both a joy and a challenge ever since Darwin's time. And as my colleagues and I have discovered, this region's best fossils are often the hardest to reach--a concept that, for reasons I will explain, has become my namesake, "Andy's Rule."
When we first visited this segment of the rugged Andes 20 years ago, we didn't know what to expect. No fossils of land-dwelling animals had ever been seen; we were drawn there by a published report of a 100-million-year-old dinosaur trackway near the summer resort village of Termas del Flaco. Little did we know that we would soon discover a much younger rock formation that preserves a trove of fossil mammals so abundant and diverse that it is now recognized as a premier archive of South American mammal evolution.
Even after we found the first bones--fragments of an ancient herbivore the size of a small horse--we knew that making additional finds would take patience and focus. Experience tells us that certain areas offer better hunting than others. All other things being equal, soils and vegetation tend to build up in valleys and on low, gentle slopes--blanketing the underlying geology and obscuring the fossil treasures they contain.
The high Andes help us around these problems, particularly when we're above the "trees" (sagey-grassy scrub brush, actually) yet still below the snow and the barren lavas of the high, young volcanoes. The trick to making a good haul is finding those special slopes where the outcrop is a cleanly exposed surface but not an inaccessible cliff. There are hundreds of miles of cliffs out there--many peppered with fossils, no doubt--but they are simply out of reach.
The various members of our expedition crews have complementary prospecting styles--some prefer methodically working through talus in gullies, some scour small regions intensively, and some cover as much ground as possible. After initial finds are made, however, we all adapt and do the careful detail work required to get the fossils out of the rock. This varied prospecting strategy--from rapid reconnaissance to meticulous scanning of promising levels--has worked very well for us, as it takes full advantage of everything that might be discovered.
My approach to prospecting is idiosyncratic. I like to hike and take in the views from up high, and I'm pretty comfortable on steep slopes. Plus, I've always had pretty good luck looking for fossils in the high spots, so that's evolved into my preferred prospecting niche.
One year, in the mid 1990s, we were working one of our more accessible central Andean localities, near Lake Teno. If the primitive road is passable, you can drive to within an hour's walk of fossils, but this place can have wicked winds. A fierce storm rolled in late one afternoon, and it began to snow. Having only a two-wheel-drive rental truck (to save precious grant money), we decided it would be smart to drop some elevation, rather than risk getting the truck snowed in for the season. So we broke camp hastily and threaded our way down this twisting road, losing some 1,500 meters in the process. We spent the night in an abandoned ranch house, the sheets of its loose corrugated metal roof clanging in the storm like gigantic cymbals--all night long.
By morning the rain had stopped, and the clouds lifted enough that we could catch fleeting glimpses of the surrounding mountains, all of them virgin territory paleontologically. Not ideal conditions, but good enough, particularly since this was our last day of fieldwork that season. We packed a lunch, set on a meeting time for the end of the day, and up into the "hills" we spread, everyone free to try their luck wherever they felt drawn. I trudged up, first over a long grassy rise, then a short flat section and a steep snow field, finally reaching a precipitous outcrop. Fortunately, after all that work, the rocks had that certain je ne sais quoi.