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.
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> 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.
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> 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 D
akotas. 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 collided with the Earth about the time the last of the great reptiles disappeared. The evidence suggests that some 65 million years ago, a hunk of rock 6 miles (10 kilometers) in diameter crashed into the ocean off the present-day Yucatan coast with the force of a 100-million-megaton bomb, creating a worldwide holocaust of toxic vapor and soot. Beads of sizzling hot glass were ejected from the blast, possibly setting the rain forests and swamps alight as far north as Saskatchewan and Alberta. Swirling clouds of ash and dust blotted out the sun, as the world settled into the gloom of “impact winter.”
Yet despite the magnitude of this disaster, time has removed almost every trace. Today—apart from a 100-mile-wide (180-kilometer-wide) crater off the Mexican coast—little remains except for a narrow band of whitish clay that is visible in several dozen places around the world. Not only does this layer contain residues of the rain of molten glass, it is also distinguished by the presence of “shocked quartz” (grains of sand that have been distorted by an impact) and by a high concentration of iridium, an element that is rare on Earth but common in meteorites and other space objects.
Once derided as sci-fi fantasy, the idea that a giant meteorite struck the Earth is now generally accepted by experts. But is that really how the large dinosaurs met their end? New findings suggest that the answer may be “yes.” Analysis of fossilized pollen from in and around the impact zone appear to reflect a sudden die-off of photosynthetic plants. Perhaps, in the dark days after the collision, food webs collapsed and great beasts like T-Rex and Triceratops starved to death. Key sites for answering these intriguing questions include the Cretaceous/Tertiary, or K/T, Boundary strata in the Red Deer Valley at Drumheller, Alberta; the Frenchman Valley in southwestern Saskatchewan; Dogie Creek in eastern Montana; and Badlands National Park in South Dakota.
* * *
High and Dry
Not long after the last dinosaur drew its final breath, something strange began to happen along the western margin of the Great Plains, in the heart of present-day Montana and Wyoming. About 50 million years ago, for reasons that no one can explain (more crashing and grinding off the west coast?), the level plains of the Cretaceous seabed began to heave upward, bend, and in places, crack open. Molten rock from the asthenosphere bubbled up through the fissures, sometimes crystallizing before it reached the surface, sometimes pouring out across the land to form dykes, domes, and ridges of lava. When the smoke cleared, mountains stood right out in the middle of the level plains.
Subsequently honed by erosion, these unexpected rocky peaks still punctuate the western landscape from the Sweet Grass Hills east through the Bear Paws and the Little Rocky Mountains, and south to the Crazy Mountains and the Black Hills.
At the time of their formation, the isolated “prairie mountains” did not have the presence that they do today. Even the main ranges of the Rockies were little more than bumps that protruded above a muddy, gravel-strewn landscape. The higher the mountains had thrust themselves up, the faster erosion had worn them away, until they lay buried, neck deep, in their own shed silt, sand, rock, and clay. (The thick coal deposits in the Powder River Basin of northeastern Wyoming were formed when tons of this muck overran a peat bog some 50 million years ago and buried the vegetation under 10,000 feet, or 3,000 meters, of sediment.) Year after year, rivers carried a massive tonnage of this debris eastward to the central plains, depositing it onto
a broad, eastward-sloping alluvial fan. As the braided streams of the floodplain washed over the sediments, they gradually licked the surface smooth, creating a landscape that in places is so level that it almost seems supernatural. This stunning flatland once extended from the knob-peaked Rockies across southern Alberta and Saskatchewan, south through the eastern Dakotas, east to the Flint Hills of Kansas, and down to central Texas. Today, though much diminished by erosion, this landscape persists as the High Plains of Nebraska, Kansas, Oklahoma, and Texas. Legend has it that when the Spanish first crossed the plains of Texas in the 1500s, they used stakes to mark their route because the land was so spectacularly featureless. Hence the name Llano Estacado, or the Staked Plains, of northern Texas.
The mysterious boulders, or concretions, at Red Rock Coulee near Medicine Hat, Alberta, were formed on the floor of a shallow, inland sea about 75 million years ago, during the Cretaceous Period.
Some 45 million years ago, when the High Plains landscape was still being shaped, it would have taken more than stakes to help travelers find their way, for it was covered by a dripping, tangled forest. Globally, the climate had never been more amenable to life—there were dawn redwoods near the North Pole—and the plains basked in warm, wet, subtropical weather. A lush woodland spread across the midcontinent, alive with an impressive variety of birds and mammals. Ancestral squirrels and monkeys leaped through the overstory, while down below, titanotheres—beasts the size of rhinos, with knobby horns and sharp tusks—shuffled across the forest floor feeding on shrubs. Among the other browsing animals of the time was an early ancestor of the horse, Orohippus by name, which had four toes on its front feet and three on the back and grew to be about the size of a large Shetland sheepdog.
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