Читать книгу Prairie - Candace Savage - Страница 23

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 sustain a thick, salty soup of algae, bacteria, and other simple forms of life. Some of these tiny creatures were capable of photosynthesis, using energy from the sun, along with carbon dioxide and hydrogen, to form glucose. When these organisms (and the others that relied on them for food) died, their energy-rich remains filtered down to the ocean floor, sometimes in and around abandoned reefs and shell middens. Here, entombed in layers of clay, they were eaten and partially digested by bacteria. Eventually, time and heat completed the transformation of solar energy into crude oil. As the black muck migrated through porous layers of rocks, it eventually found its way into reservoirs, where it collected. The pump jacks at work on the plains today are sucking up wealth that was created between 450 million and 100 million years ago.

> FOSSIL HOT SPOTS ON (AND NEAR) THE PRAIRIES

> AGATE FOSSIL BEDS NATIONAL MONUMENT, Gering, Nebraska, features fossils of rhinos, bear-dogs, land beavers, and other animals that lived on the savannas about 20 million years ago.

> BADLANDS NATIONAL PARK, Interior, South Dakota, is rich in fossil mammals, including early rhinos, horses, pigs, and camels that date to between 23 million and 37 million years ago. A much older Tyrannosaurus rex skeleton, nicknamed Sue, was found nearby.

> BURGESS SHALE, Yoho National Park, Field, British Columbia, contains a stunning sample of the sea life that would have flourished on the plains (then the continental shelf) just over half a billion years ago.

> DINOSAUR PROVINCIAL PARK WORLD HERITAGE SITE, Patricia, Alberta, provides access to a stretch of the Red Deer Valley from which the skeletons of more than three hundred Cretaceous dinosaurs have been recovered. Many of them are housed in the Royal Tyrrell Museum of Paleontology in Drumheller, Alberta.

> DINOSAUR NATIONAL MONUMENT, near Dinosaur, Colorado, and Jensen, Utah, preserves the remains of Jurassic dinosaurs—the original Jurassic Park.

> GUADALUPE MOUNTAINS NATIONAL PARK, Salt Flat, Texas, features a 250-million-year-old reef that once lay on the bottom of the sea and now towers over the deserts and plains of west Texas.

> HOT SPRINGS MAMMOTH SITE, Hot Springs, South Dakota, contains the fossilized remains of at least fifty-two mammoths and other Ice Age creatures.

> LUBBOCK LAKE LANDMARK, Lubbock, Texas, preserves evidence of human activities (including hunting) over the last 11,500 years.

> PICKETWIRE CANYONLANDS DINOSAUR TRACKSITE, Commanche National Grassland, near La Junta, Colorado, still bears the imprint of dinosaurs that plodded through the mud 150 million years ago.

> T-REX DISCOVERY CENTER, Eastend, Saskatchewan, focuses on one of the most complete Tyrannosaurus fossils ever uncovered.

> WYOMING DINOSAUR CENTER, Thermopolis, Wyoming, is devoted to the study of Jurassic dinosaurs, such as Allosaurus and Diplodocus.

Beaked synapsid

Dimetrodon

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 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.