Imagine the badlands of eastern Montana, a stark, heavily eroded landscape of steep-sided coulees, sandstone outcrops, and boulder-strewn washes. Vegetation is sparse, little more than sage, bunchgrass, scattered islands of scrub pine, and an occasional yucca plant. It's here, south of Fort Peck Reservoir, on a sunny spring afternoon, that you could find Jack Horner lying on his belly atop a barren hill in the Hell Creek Formation, one of the world's most famous dinosaur graveyards. The paleontologist is a big man, standing 6 feet 4 inches tall, but he might just as well be a boy again, chancing upon his first dinosaur fossil, so thoroughly absorbed is he in exploring the tiny objects only inches from his face. Horner is best known for finding large skeletons, nesting colonies, and vast bone beds where herds of dinosaurs died, and his dig here has turned up a number of outstanding specimens, including eight Tyrannosaurus rex skeletons, one of which is the largest in the world. Today he's sprawled in the dirt, patiently sorting through what may strike the untrained eye as everyday detritus but which might hold clues to a new understanding of why the dinosaurs perished. What exactly has captured his attention?
The technical term is micro-site, which means an assemblage of small fossils, some so tiny they cannot be seen without a microscope. Micro-sites often contain remains from a variety of organisms, and paleontologists use them to reconstruct a slice of everyday ancient life. At this location, for instance, Horner and two colleagues have collected lizard skull fragments; shards of turtle shell; fish teeth, scales, and vertebrae; even a handful of dinosaur toe bones. Remnants of snails, clams, frogs, crocodiles, rodent-size mammals, and other animals have been excavated in neighboring micro-sites. The fossils are found together because they were once transported by a swift-running creek or river and deposited as the flow of water slowed down or came to a standstill. What makes these miniature menageries important is that they include species that lived in or near a particular stream at the same time. ""Just by picking through one of these sites,"" Horner explains, ""we can figure out which animals occupied the same ecological space.""
Exploring ecological space on an unprecedented scale has brought Horner to this remote part of the American West. He and the dozen senior scientists he has assembled for this expedition are reconstructing the ancient ecosystem of the Hell Creek Formation. Perhaps the most significant of the many interests of Hell Creek is what the formation reveals about how that world came to be lost. Whereas the age of the bottom layers is somewhat uncertain, that of the top, or most recent, layers is known precisely—64.5 million years old. That date marks the so-called K-T boundary, when the Cretaceous Period ended and the Tertiary Period began, and when the 160-million-year-long saga of the dinosaurs came to a close. In other words, the Hell Creek Formation represents the final scenes in the third act of one of evolution's most successful dramas. And the events recorded in this suite of sedimentary rock suggest that the common explanation for the extinction of the dinosaurs—a massive asteroid impact—doesn't fully account for their undoing. What's more, the Hell Creek Formation may tell us something about the fragility of life today.
