half of North America. Apparently, the asthenosphere had heated up and begun to force masses of molten rock up through rifts in the ocean floor. This event had sent the continental plates on a slow and perilous collision course. First, Europe smashed against North America from the northeast. Then a massive supercontinent called Gondwana (the combined landmasses of South America, Africa, India, Antarctica, and Australia) crunched into North America from the south, causing the land to buckle and forcing the Appalachian Mountains to lift along the east coast. The forces involved in these mighty adjustments were even felt in the middle of the craton, where a range of mountains 3,000 feet (900 meters) high rose out of the plains of present-day Oklahoma and Colorado. Known as the Ancient Rockies, these mountains have since been eroded to their roots by the action of water and wind.
Apart from the appearance of these new highlands, the west coast of the craton was comparatively untouched by these titanic collisions. Through all the commotion, the sea continued to wash up over the land, even splashing around the base of the Ancient Rockies and turning them into a cluster of islands. With every advance and retreat, the sea again left behind layers of sediment and the fossilized remains of a strange coterie of underwater life. In addition to the crinoids, corals, and other unusual beasts that had occupied Devonian waters, there were now small filter-feeders, called archimedes, that had perfect corkscrew skeletons, and others, called productids, that held themselves up off the sea bottom by perching on stiltlike spines. (A wonderful jumble of 250-million-year-old sea life has been preserved in the Guadalupe Mountains of western New Mexico and Texas, which were once a complex of reefs in the western ocean.) Bony fishes swam through these waters, sometimes hotly pursued by large, saw-toothed sharks. The game of evolution was being played with feverish exuberance.
Nautiloids
Productid
Meanwhile, back on dry land, the surface of the continent was continuing to buckle and twist. As the Appalachians were thrust upward, land in the center of the craton was forced to rise along with them. A broad plain formed along the edge of the eastern highlands, sloping gently toward the western sea. When the waters receded, this coastal plain extended all the way west to present-day Alberta and Texas. And even when the sea rose up and flowed across the land, the eastern margin of the plain (roughly from present-day Manitoba south to Kansas and Missouri) was now high enough to escape all but the most severe flooding.
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
