| This site is for the Mesozoic Era | ||||||||||||||||||||||||
| The Cretacious Period | ||||||||||||||||||||||||
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Cretaceous Period, in geology, latest time period of the Mesozoic era, lasting from about 138 million to about 65 million years before present. The name alludes to the abundance of chalk (Latin creta) strata deposited during the latter part of the period in England and France and now exposed at sites such as Dover. In Europe and North America, geologists divide the period into an Early and a Late Cretaceous. At the beginning of the Mesozoic era, all the continents had been joined as one landmass, Pangaea. The breakup of Pangaea in the early Mesozoic created two supercontinents: Laurasia, consisting of what are now the northern continents; and Gondwanaland, consisting of the southern . Between the two lay a vast sea, the Tethys, of which today's Mediterranean is a greatly shrunken remnant. In the Cretaceous period the African continental plate broke from Gondwanaland and drifted north, subjected Tethys Sea sediments to powerful compressions, creating the roots of the European Alps. Later the African plate plunged beneath the Laurasian one, triggering the volcanic activity that persists today in Italy and Sicily. Meanwhile the newly formed South Atlantic Ocean widened due to seafloor spreading along the Mid-Atlantic Ridge, broadening the gap between Africa and South America. Farther east, India had separated from Gondwanaland and, drifting north, bucked eastern Tethys sediments into elongate ridges that were the forerunners of today's Himalayas. Antarctica and Australia, still joined, drifted south and westward. The continuing westward movement of North America generated mountain-building forces that culminated in the uplift of the Rocky Mountains and California's Sierra Nevada. The rising Rocky Mountains blocked westward draining of the advancing Late Cretaceous sea, turning much of the western interior of North America into a vast swamp. In the east, sediments produced by erosion of the Appalachian Mountains formed the Atlantic coastal plain. During the Late Cretaceous, sea levels rose worldwide, submerging about one-third of the earth's present land area. This allowed heat from the sun to be distributed farther poleward by ocean currents, producing a warm, mild global climate with ice-free poles and Arctic water temperatures of 14� C (58� F) or higher. In such a climate, cold-blooded reptiles could exist even in northern latitudes. Fossil ferns and cycads found in Cretaceous rocks at Arctic latitudes are similar to plants growing today in subtropical rain forests. By the Late Cretaceous period the flora had taken on a modern appearance and included many of today's genera of trees, such as the oak, beech, and maple. Despite these mild conditions, several mass faunal extinctions occurred toward the end of the period. Five great reptilian groups-dinosaurs, pterosaurs, ichthyosaurs, plesiosaurs, and mosasaurs -that had been dominant became extinct. A recent theory is that a comet or small asteroid colliding with earth 65 million years ago blasted enough dust into the atmosphere to reduce incoming solar radiation and temperatures worldwide, devastating the algae, vegetation, and small animals on which the large reptiles depended for food. | |||||||||||||||||||||||
| The Jurrassic Period | ||||||||||||||||||||||||
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Jurassic Period, second division of the Mesozoic Era of the geologic time scale (see Geology), extending over a period from about 208 to 144 million years before present. The period is named for strata from the Jura Mountains. At the beginning of the Jurassic Period, Italy, Greece, Turkey, and Iran were attached to the North African portion of Gondwanaland, the southern supercontinent that had begun to break up during the Triassic Period. Antarctica and Australia, already detached from Gondwanaland, remained joined together, while India drifted northward on a collision course with the supercontinent Laurasia. North America tore free from Gondwanaland and drifted westward. This opened the Gulf of Mexico, from whose waters thick salt beds would be deposited. As North America overrode the floor of the Pacific, it triggered the volcanism and the intrusion of batholiths (large, subterranean bodies of granitic igneous rock) that began the mountain-building events that would later culminate in the rise of the North American Cordillera (the Rocky Mountains and the Sierra Nevada). Meanwhile, in the southern hemisphere, South America and Africa began rifting apart, and a long, narrow seaway worked northward between the two continents, eventually joining with the great equatorial sea called Tethys. As seaways grew and joined, warm, shallow marine waters spread over much of Europe and other landmasses that bordered the Tethys Sea. Toward the end of the Jurassic Period, these shallow seas began to drain away, leaving thick limestone beds where some of the world's richest accumulations of oil and gas would form. Evidence that the Jurassic climate was warm and moist is provided by widespread coral reefs and by the remains of temperate and subtropical forests consisting largely of the gymnosperms (cycads and conifers), ginkgoes, and seed ferns. Angiosperms (flowering plants) first appeared in the mid-Jurassic Period. Reptiles were the dominant form of animal life. They had adapted to life in the sea (ichthyosaurs and plesiosaurs), in the air (pterosaurs), and on the land, where some reached huge sizes (the carnivorous Allosaurus and herbivorous Apatosaurus). In 1822, Jurassic strata in Sussex, England, yielded the bones of one of the first dinosaurs discovered, Iguanodon. Fossil remains of the oldest known bird, the toothed Archaeopteryx, were also found in rocks of Jurassic age. Mammals, which had evolved at the end of the preceding Triassic Period, remained small and rodentlike while the dinosaurs flourished throughout the Jurassic Period. | |||||||||||||||||||||||
| The Triassic Period | ||||||||||||||||||||||||
Triassic Period, first of the three divisions of the Mesozoic era of the geologic time scale (, spanning an interval of 35 million years, from 240 to 205 million years ago. | During the Triassic period, the supercontinent Pangaea began tearing apart . Rifts thus developed between North America and the African portion of Gondwanaland. As the earth's crust stretched, large blocks subsided, creating physiographic basins in which thick beds of red-stained sandstone, siltstone, and shale were deposited. These sedimentary rocks were intruded by sills of dark, igneous basalt, such as the one exposed in New Jersey's Palisades, along the west bank of the Hudson River. Narrow elongated remnants of the Triassic basins occur from Virginia north to Nova Scotia. Fossils found in Triassic strata indicate that the general climate was warm at that time. Much of North America lay between the equator and 30� North latitude, so that subtropical conditions existed as far north as Wyoming and New England. The terrain of Triassic times was dominated by evergreen trees, most of them conifers and ginkgoes. The cycad palms and scale trees that were the most prominent flora of the preceding period still existed, but were not as numerous or varied as the evergreens. The most important animals to make their appearance in the Triassic were the dinosaurs . The earliest members of this group did not attain the huge dimensions of the dinosaurs of later Mesozoic times and were for the most part no more than 3 to 4.5 m (10 to 15 ft) long. Other important reptilians of the Triassic were the ichthyosaurs-marine reptiles with long snouts and with bodies shaped like those of modern dolphins. Other marine reptiles of the period, the plesiosaurs, had broad, turtlelike bodies, long necks and tails, and large flippers. The Triassic period is considered by many paleontologists to mark the emergence of the first true mammals, but little is known of their physiology. Among the invertebrates, insects were represented in the Triassic by the first species to undergo complete metamorphosis from larva through pupa to adult. In the Triassic seas, where squidlike belemnites, ammonites, and crustaceans flourished, 75 percent of invertebrate species were obliterated by a mass extinction.
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