Enchanted Rock, near Austin, Texas, was formed as a massive upwelling of molten rock during the Precambrian Era. The granite eventually solidified and now lies exposed by the eroding action of wind and water.
Nautiloids
Crinoid/sea lily
And so things continued for about the next 100 million years, as shallow oceans advanced across the North America craton, only to withdraw and then flood back in. If the run and roll of the grasslands sometimes remind us of the sea, surely this is a result of the landscape’s long marine history. With every advance and retreat of the ocean, the land was burdened with fresh deposits of sand, silt, and crushed shells, which built up, year by year, in nearly horizontal, banded layers. Although some of these contrasting sea floors have since been exposed by erosion (where rivers have cut deeply down through the sediments), for the most part they lie thousands of feet beneath the grasslands.
The oceans that left these deposits behind were hospitable to life—shallow, warm, well lit, and typically tropical. During the Silurian and Devonian periods in particular (between about 440 million and 355 million years ago), these waters provided ideal conditions for reef-forming sponges and corals. In what would one day become the Canadian Prairie provinces, the reef builders of the Devonian had a heyday, constructing barrier reefs and ringlike walls that rose to heights of 300 feet (100 meters). Wherever the sea was constricted by these limestone palisades, the water gradually became super-salty. If the circulation of the sea was inhibited, water lost to evaporation could not readily be replaced, and the concentration of salts steadily increased. In time, the salts precipitated out of the sea water in these areas, leaving thick beds of potash and other minerals, notably under present-day Saskatchewan. The potash deposits in Texas were formed by a similar process but some millions of years later, during the Permian Period.
The last truly continentwide inundation withdrew from the North American craton about 300 million years ago. The next time the sea attempted to overrun the land—as it would continue to do for millions of years to come— it found itself lapping around the shores of a rocky upland that had started to rise in the eastern 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 land masses 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 (1,000 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 south-sea 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.
Archimedes
Productid
Meanwhile, back on dry land, the surface of the continent was continuing to buckle and twist. As the Appalachians were thrust upwards, 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.
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.
> FOSSIL SUNBEAMS
The modern world runs on energy that originally beamed down from the sun millions of years ago. During the eons when tropical seas lapped over the North American plains, the sun provided heat and light to
