Jurassic period
Jurassic period [from the Jura Mts.], second period of the Mesozoic era of geologic time, lasting from 213 to 144 million years ago. At the start of the Jurassic most of the continents were joined together until the Atlantic began to form and the Americas split off from Africa. Eastern North America was mostly elevated and subject to erosion, which reduced the Appalachian region to a peneplain. Before the end of the period, the Appalachian borderland began to founder as the Atlantic Ocean continued to widen. The Pacific border of North America, from California to Alaska, was submerged for most of the period. In the Early Jurassic, large areas of Arizona, Colorado, and Utah were apparently desert, and the sand was later consolidated into the white and pinkish Glen Canyon and Navajo sandstones, which now enhance the scenic beauty of the district. During the Upper Jurassic, the Logan Sea entered this area from the north. In its various advances and retreats, this body of water covered large areas of Montana, Idaho, Wyoming, Colorado, and Utah, depositing sandstone, shale, limestone, and some gypsum. The retreat of the Logan Sea, toward the end of the period, was followed, probably in the Upper Jurassic but possibly in the Lower Cretaceous Period, by the deposition of the Morrison continental series of clays and sandstones, noted for its richness in fossil dinosaurs. The close of the Jurassic in North America was marked by widespread folding along the western border of the continent, accompanied by the intrusion of lava as the eastern edge of the plate that carries the Pacific Ocean was thrust beneath the westward drifting plate that carries the North American continent. In this disturbance the Sierra Nevada, Klamaths, Cascades, Coast Ranges, and coastal mountains of Canada and Alaska were formed. The history of the European Jurassic is very well known, that system being one of the most complete on the Continent. Studies of oxygen isotopes, the extent of land flora, and marine fossils indicate that climates during Jurassic times were mild—perhaps 15°F (8°C) warmer than those of today. No glaciers existed during this period. The plant life of the Jurassic was dominated by the cycads , but conifers , ginkgoes , horsetails , and ferns were also abundant. Of the marine invertebrates, the most important were the ammonites. The dominant animals on land, in the sea, and in the air were the reptiles. Dinosaurs , more numerous and more extraordinary than those of the Triassic period, were the chief land animals; crocodiles, ichthyosaurs, and plesiosaurs ruled the sea, while the air was inhabited by the pterosaurs and relatives. Mammals, making their first appearance, were few and small but undoubtedly became well established during the Jurassic period. The Jurassic saw the appearance of the first bird, Archaeopteryx. See Geologic Timescale (table).
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