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.
Top of theTorosaur
One of the prize discoveries at Montana's Hell Creek Formation is the largest dinosaur skull ever found. The 9-foot-long skull belonged to a mature torosaur. Adults of this species probably reached at least 25 feet in length and may have weighed 4 to 5 tons. Notes on the edge of the Polaroid point out the eye (orbital horn), nasal horn, nostril, and frill, or neck shield. Although torosaurs resembled Triceratops, says Horner, they have holes in the frill that Triceratops lacked. Torosaur means ""piercing reptile"" in Latin.Blessed with broad expanses of fossil-bearing rock that were exposed and scoured when the last continental ice sheet retreated 18,000 years ago, eastern Montana appears early on in the development of dinosaur paleontology. In 1855 Ferdinand Vandiveer Hayden, a geologist with the United States Geological Survey, found a variety of teeth just upstream of Fort Peck Reservoir where the Missouri and Judith Rivers meet. They were later identified as belonging to dinosaurs, the first such discovery in the Western Hemisphere. Subsequent excavations in the region during the second half of the 19th century turned up nearly complete skeletons of duck-billed, horned, and armored dinosaurs. Then in 1903, while surveying the Hell Creek Formation, a collector from the American Museum of Natural History named Barnum Brown came upon the remains of an extraordinary animal no one had seen before— Tyrannosaurus rex. Parts of a second T. rex were unearthed in the same area five years later. Since then, several other skeletons have been discovered, including one in 1988 known as the Wankel T. rex, the first specimen to include the animal's tiny two-clawed arms.
Now three years into the five-year-long Hell Creek Project, Horner, his crews, and volunteers have picked up the pace of discovery. Their finds include the world's largest duck-billed dinosaur, the largest dinosaur skull ever excavated, and approximately 60 separate partial remains of Triceratops, the three-horned behemoth common to this area of North America at the time. Most impressive, however, are the tyrannosaurs. One is at least 10 percent bigger than Sue, the controversial South Dakota dinosaur that heretofore was the largest T. rex ever uncovered. Another, exquisitely preserved, is the oldest. Yet another is the smallest in the world, meaning it is a juvenile, which can lead to an understanding of how tyrannosaurs matured.
Fascinating as the dinosaurs may be, however, they form only one part of the comprehensive picture here. Horner and his experienced colleagues—a structural geologist; a stratigrapher; a taphonomist (one who studies what happens to animals after they die); paleontologists specializing in vertebrate, mammalian, plant, and mollusk fossils; a molecular paleontologist; and an expert on paleomagnetism—are surveying all the fauna and flora that existed during the Hell Creek period (and that survived as fossils), the ways they interacted, and how they may have evolved. In addition, they are looking for traces of rivers, lakes, and saltwater bays, which will reveal the region's climate over the course of 2 to 3 million years.
Convention has it that the Hell Creek Formation represents an uneventful period in the planet's history. During the Cretaceous Period, the oceans rose several times, flooding the continents. One part of that inundation was the Western Interior Seaway, which stretched from the Gulf of Mexico well into Canada and divided North America in two. For tens of millions of years, the seaway retreated and advanced, its western edge once advancing within 50 miles of the Rocky Mountains. Just before the Hell Creek sediments were deposited, about 68 million years ago, the seaway withdrew for good, leaving behind the configuration of continent and surrounding oceans that exists today. During the 2 to 3 million years that followed, many scientists agree, both the environment and the animal and plant populations of the region remained stable.
All that changed drastically and violently 64.5 million years ago when a giant asteroid slammed into the planet north of the Yucatán Peninsula, apparently creating a worldwide pall of dust, ash, and debris. During the next three to six months—some have postulated an aftermath that lasted up to a year—of darkness, freezing cold, and perpetual acid rain, a catastrophic die-out occurred, affecting both marine and terrestrial organisms. Among the many casualties were dinosaurs.
