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.
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
