Читать книгу Prairie - Candace Savage - Страница 25

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.