Labels:text | black and white | font | screenshot | document OCR: The Cretaceous Period (144-65 MYA) marked the last phase of the Mesozoic Era and the climax of the Age of Dinosaurs. During this period, thick chalk beds covered the floors of shallow seas that invaded North America and Europe; Laurasia and Gondwana fragmented into the continents that we know today; and colliding slabs of continental and oceanic crust pushed up the Rockies and other mountain ranges. Close to the equator, climates stayed warm but became drier, and forests thinned out. Elsewhere the seasons grew more marked, and flowering plants appeared. Animals on the now-isolated continents started to evolve separately from one another; this explains why more dinosaur species appeared in Cretaceous times than ever LANDS Early in the Cretaceous Period, a land bridge linked eastern Asia and western North America, forming one region known as Asiamerica. Gradually during the Cretaceous Period, a shallow sea split North America in half, and western North America became an island with its own dinosaur species. A shallow sea separated Euramerica (eastern North America, Europe, and Greenland) from Asia. Also, the southern lands of Africa, Antarctica, India, South America, and Australia gradually drifted apart. As the widening Atlantic split eastern North America from Europe, Europe was left with a mixture of "advanced" northern dinosaurs and "old-fashioned" southern types. PLANTS Mesozoic plants, such as cycads (palmlike tropical plants), ginkgoes (trees with fan- shaped leaves), conifers, and ferns, still dominated the land in early Cretaceous times. Small, weedy, flowering plants (angiosperms) started to appear; as these prolific plants spread from the tropics into cooler regions, some evolved into shrubs and small trees. Eventually trees resembling modern oaks, maples, walnuts, and other trees were competing with the still-abundant conifers. LIFE ON LAND By late Cretaceous times, small mammals (animals that have hair and feed their young on milk) were diversifying fast, and snakes had evolved from lizards, but dinosaurs still dominated life on land. New plant-eating ornithischian ("bird-hipped") families appeared, equipped with teeth and jaws that were designed for chewing the new flowering plants. Low-browsing ankylosaurs (a group of armored dinosaurs) multiplied too, but the less well-defended stegosaurs (four-legged dinosaurs with two tall rows of plates or spines running down the neck, back, and tail) died out. The great, long-necked sauropods ("lizard-footed" dinosaurs) began to diminish in northern lands, although they still predominated in the south. Cretaceous theropods ("beast-footed" dinosaurs) ranged from birdlike ornithomimids ("bird-mimics") to some of the largest of all land flesh-eaters, the tyrannosaurids. LIFE IN THE AIR Winged creatures of Cretaceous times ranged from the world's first tiny moths and small social bees to the biggest animals that ever flew: giant pterosaurs (flying reptiles). Among the largest pterosaurs were Quetzalcoatlus and Pteranodon. New finds published in the early 1990s show that birds were also diversifying. By the early Cretaceous Period, little Sinornis from China and Concornis from Spain both had a ridged breastbone, like modern flying birds. Later, diving and running birds that did not fly appeared, among them the turkey-sized Mononykus, with a ridged breastbone and a long, bony, dinosaurian tail core. LIFE IN WATER Cretaceous water creatures included immense relatives of living vertebrates (animals with a backbone). The largest freshwater species were crocodilians (crocodiles, alligators, and related extinct reptiles) and the huge, crocodile-like champsosaurs, while the marine turtle Archelon outgrew the largest leatherback living today. Seagoing, flippered lizards, known as mosasaurs, grew to 10 m (33 ft) in length and swam with undulations of a deep, flattened tail. The flippered, swimming plesiosaurs (giant sea reptiles with a bulky body and large, paddlelike limbs) included such short-necked forms (called pliosaurs) as the formidable Kronosaurus, with a head 3 m (10 ft) long, and long-necked fish-eaters, such as Elasmosaurus, which had no fewer than 70 neck vertebrae (back bones). During this period, ichthyosaurs (streamlined, dolphinlike reptiles with fins, flippers, and long, narrow jaws) died out.
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