A new frontier for life was emerging not only in North America but on the other continents as well. Land plants, which had put in their first appearance some millions of years before, had never made much of a showing. But as stable new habitats became available, the evolutionary tree began to bud and sprout with explosive energy, producing more and larger species of land plants than ever before. In time, the soggy, boggy landscapes left by the retreating oceans were filled with riotous jungles of giant sphenopsids, or scouring rushes, tree-sized ferns and leafy conifers. These tremendous swamps, which flourished between about 355 million and 300 million years ago, disappeared soon afterward, probably as a result of a cooling and drying trend in the climate. Buried where they fell—in modern-day Iowa, Missouri, and Kansas, among other places—the swamp plants eventually turned into coal, the characteristic rock of the Pennsylvanian, or Upper Carboniferous, Subperiod.
Through much of the next 50 million years (the Permian Period), the land shriveled in the sun. Swamps decayed, seas shrank, and the exposed plains along the west coast blew with sand and salt. But life was not to be stopped. Insects, which had dominated the wetlands of the Carboniferous, now gave rise to new dry-land forms such as beetles and the distant ancestors of crickets and grasshoppers. Amphibians, too, crawled out of the swamps and began to invent the technology they needed for life on the land—notably a soft-shelled, amniotic egg that could develop out of the water. In time, new life-forms developed that could live their whole lives on land, including massive, lizardlike creatures known as stem reptiles. Basking alongside these primitive organisms on the arid coastal plains were their near relatives, the synapsids—the direct ancestors of modern mammals. At the root of our family tree is Dimetrodon, a burly, fin-backed synapsid with two stabbing canine teeth, which it used to snap up slow and unwary amphibians. We know these creatures once roamed the savannas of the western plains because wonderfully preserved skeletons of Dimetrodon and many of its equally bizarre contemporaries have been dug out of Permian “red bed” deposits in New Mexico, Texas, and Oklahoma.
Dimetrodon
And so it is that we find ourselves near the end of the Permian, watching a lumpish, beaked synapsid called Kannemeyeria breaking off the tough stem of a broad-leafed conifer somewhere along the west coast of Texas. Under our feet lie the accumulated sediments of 3.5 billion years, or more than 90 percent of the geological timeline. Yet except for the wide spread of the horizon, there is little in this scene to put us in mind of the modern prairies. No grass, no gophers, no pronghorns, no playas or sloughs. Something radical will have to happen to create the landscape that we see around us today. Something revolutionary.
TERRIBLE LIZARDS
THE PERMIAN PERIOD ended in a biological catastrophe—the most severe mass extinction in all of geological history. During a period of several million years, over 95 percent of all the species living in the oceans were eliminated, 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 landmass, 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 EARTH IN UNSTEADY MOTION
Sometime in the first half of the nineteenth century, a man named Little Hill, “a Winnebago chief from the upper Mississippi, west,” shared this story with a U.S. government official and amateur ethnographer named Henry Schoolcraft. Little Hill’s narrative reflects the beliefs of the Winnebago Buffalo Society about the creation of the world.
Little Hill on Creation
The Great Spirit awoke as from a dream, and found that he was alone. He created the four winds by taking a piece of flesh from near his heart and mixing it with the substance upon which he sat. For these brothers he created a woman, our Grandmother the earth. She was sent down below, but she was unstable, and rocked about violently. To steady the world below, the Great Spirit sent down four giant snakes and four giant animals of another kind, and they were able to hold down the corners of the earth. However, when the winds blew across this creation, it fell back into unsteady motion again; so he created a gigantic buffalo, who is the land, and placed it in the center of the earth to make it steady.
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 east into 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 a titanic struggle among rivers, mountains, and seas. They also contain evidence of an awe-inspiring bestiary of ancient life.
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 currents and dropped on snags and in backwaters, where they lay in thick beds. These Morrison deposits provide the focus for the Dinosaur National Monument in Colorado and Utah. Although now in the mountains, the deposits formed on level terrain, 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.
The sea, which in the Late Jurassic had been driven off the continent by sediment from the mountains to the