Prairie. Candace Savage

together with 75 percent of terrestrial vertebrates. Why did this disaster occur? No one knows for sure, but the continuing gyrations of the continental plates may have been partly to blame. By the Permian Period, the continents had become temporarily fused into one gigantic land mass, called Pangaea. At the same time, the floor of the oceans apparently warped downward, drawing the sea away from the land and exposing a vast and inhospitable heartland of hot, dry silt and sand. These deserts had little to offer to life-forms that, in ages past, had flourished in a watery world of lagoons and swamps. Perhaps this change in conditions is enough to explain the huge loss of life. But whatever the probable causes (and many have been invoked), the impact was severe, and, despite the nonstop creativity of evolution, it would take millions of years for the Earth to repopulate itself with a full range of plants and animals.

      At the same time that this biological revolution was occurring, a major geological upheaval was also underway. The continents, after docking together in Pangaea for some millions of years, began to tear away from one another. As Europe sheared off to the east and the Atlantic Ocean opened up, the North American craton was shoved slowly westward. Eventually, about 165 million years ago, the drifting continent ran into a small fragment of the Earth’s crust (perhaps an island chain), known to geologists as a terrane. As the continent plowed onward, it contacted other, similar obstacles in its path. One by one, these terranes were crushed against the west coast of the craton and added to its mass. The impact of these collisions—which would continue sporadically for about the next 100 million years—caused the western margin of the craton to fold, twist, crack, and rise up mightily, until ranges of ragged peaks ran along the length of the continent. The present-day plains (which for so long had lain along the west coast, exposed to the run of the sea) were now guarded by the serried ranks of the Rockies.

      The earliest amphibians appeared about 345 million years ago, as long-bodied, short-limbed animals that resembled crocodiles with finned tails. Modern frogs and toads put in an appearance somewhat later, alongside the dinosaurs of the Jurassic Period.

      Early in this process, before the wall of mountains was complete, the ocean still sometimes slipped through gaps in the palisade and washed across the plains. This happened several times during the Triassic and Jurassic periods (between 250 million and 145 million years ago), culminating in a huge incursion, known as the Sundance Sea, which swept as far east as present-day Saskatchewan, Nebraska, and Texas. But these waters were soon expelled from much of their floodplain by a deluge of a completely different sort—an influx of mud and sand that washed down off the slopes of the newly formed mountains. No sooner had the mountains raised their heads than erosion began to level them. Mixed with generous quantities of volcanic ash from the tumult of mountain building, these sediments were strewn across the plains as far east as the Dakotas. Today they form brightly banded sandstones and shales—the Success, Kootenay, and Morrison formations by name—that bear witness to an awesome struggle among rivers, mountains, and seas. They also contain evidence of an awe-inspiring bestiary of ancient life.

      At home in the sagebrush country and short-to-mixed grasslands of the northern and central Great Plains, the greater short-horned lizard is descended from reptiles that lived during the dinosaurian era. It is a member of the iguana family and subsists largely on ants.

      The cataclysmic extinctions at the end of the Permian had left a biological void, but by the Middle Triassic (about 225 million years ago), this vacuum had been filled to bursting with reptiles. Creeping, crawling, swimming, flying, stomping across the land, reptiles had become the dominant animal group on Earth. Chief among them were the dinosaurs, including the 80. to 100-ton Brachiosaurus, which raised its ultralong neck to browse in the treetops, and the plated Stegosaurus, which had seventeen trapezoidal shields of bone embedded along its spine. Unfortunately for them (but fortunately for succeeding generations of dino-enthusiasts), hundreds of these large-bodied, small-brained animals apparently tramped into the rushing rivers, got stuck in the mud, and died. Their bones were then swept away by the current and dropped on snags and in backwaters, where they lay in thick beds. These Morrison deposits provide the focus for the Dinosaur National Monuments in Colorado and Utah. Although now in the mountains, the deposits were laid down on the plains, and the same or similar species must have lounged under ginkgo trees and trudged through the spiky underbrush of what is now the Great Plains.

      Brachiosaurus

      Stegosaurus

      The sea, which in the Late Jurassic had been driven off the continent by sediment from the mountains to the west, managed to creep in one last time during the Cretaceous Period (between 145 million and 65 million years ago). By this time, the Rocky Mountains formed an unbroken dyke along the west coast, but the rising waters rushed around it at both ends, flowing south from the western Arctic and north from the Gulf of Mexico. By the time the waters met in Colorado about 100 million years ago, almost the entire prairie region was inundated. It would take another 40 million years or so for the sea to make its final retreat, but when the water was finally gone, a vast plain lay exposed, stretching farther than the eye could see across the interior of the continent.

      The first eyes to gaze across those broad, unfettered vistas were no doubt reptilian. By the Late Cretaceous, herds of heavyset Triceratops, with their wide, frilled collars and clustered horns, were roaming across the countryside and foraging in lush stands of horsetails, ferns, gingkoes, and palms, keeping an eye peeled for their most dangerous enemy, Tyrannosaurus rex. In case we were in any doubt about what T. rex preyed upon, a paleontologist in Saskatchewan has recently found a large sample of fossilized tyrannosaur dung. The .5-gallon (2.3-liter) lump contains what appears to be the crushed head frill of a juvenile Triceratops. Much of what we know about Cretaceous dinosaurs, both trivial and profound, comes from sites on the Great Plains, including the Red Deer River Valley in Alberta and the Frenchman Valley in Saskatchewan.

      Triceratops

      Tyrannosaurus rex

      Succored by a mild and equable climate, much improved since Permian times, life in the Cretaceous was full. The air thrummed with insects, including moths and bees. The massive flying reptile Quetzalcoatlus rode the updrafts over the southern plains on a span of 35- or 40-foot (11- or 12-meter) wings—wider than those of a single-engine plane—searching for the bodies of the dead and dying. Frogs and salamanders hid in the underbrush, a habitat they shared with cowering, timorous mammals, few of them bigger than mice. Yet by the end of the Cretaceous Period, about half of this rich assembly of species—including all of the flying reptiles and the dinosaurs—had completely disappeared. And again, the reasons for this mass extinction are unclear. Most geologists believe that the Earth was hit by a huge meteorite that obscured the sun with a thick cloud of dust. Others argue that a sequence of volcanic explosions in India and elsewhere had a calamitous cooling effect on the climate. Still others believe that a gradual deterioration of the climate, over several million years, eventually put paid to the terrible lizards. But whatever the cause or causes, the outcome was clear. The dinosaurs were gone, and the plains were ready and waiting for a new group of pioneers to make themselves at home on their wide open spaces.

       > THE THIN WHITE LINE

      Did the catastrophe that killed the dinosaurs come flaming out of the skies? For the last thirty years, geologists have been mesmerized by the possibility that a huge meteorite