Jurassic

THE JURASSIC PERIOD extended from about 199 million years ago to 145 million years ago. This geological time period constitutes the middle of the Mesozoic era, also known as the Age of Dinosaurs. The start of the period is marked by the major Triassic-Jurassic extinction event. This period was named by Alexandre Brogniart for the extensive marine limestone exposures of the Jura Mountains, in the region where Germany, France and Switzerland meet.

During the early Jurassic, the supercontinent Pan-gaea broke up into the northern supercontinent Laur-asia and the southern supercontinent Gondwana. The Gulf of Mexico opened in the new rift between North America and what is now Mexico's Yucatan Peninsula. The Jurassic North Atlantic Ocean was relatively narrow, while the South Atlantic did not open until the following Cretaceous Period, when Gondwana itself rifted apart. The Tethys Sea closed, and the Neo-tethys basin appeared. Climates were warm, with no evidence of glaciation. As in the Triassic, there was apparently no land near either pole, and no extensive ice caps existed.

The Jurassic geological record in western Europe is clear, where extensive marine sequences indicate a time when much of the continent was submerged under shallow tropical seas. In contrast, the North American Jurassic record is the poorest of the Meso-zoic, with few outcrops at the surface. Though the epicontinental Sundance Sea left marine deposits in parts of the northern plains of the United States and Canada during the late Jurassic, most exposed sediments from this period are continental, such as the alluvial deposits of the Morrison Formation. The first of several massive batholiths were emplaced in the northern Cordillera, beginning in the mid-Jurassic, marking the Nevadan orogeny. Important Jurassic exposures are also found in Australasia, India, Japan, Russia, South America, and the United Kingdom. During the Jurassic period the primary vertebrates living in the seas were fish and marine reptiles. The latter include ichthyosaurs, plesio-saurs, and marine crocodiles, of the families Teleo-sauridae and Metriorhynchidae. In the invertebrate world, several new groups appeared, including rud-ists and belemnites. The Jurassic period also had diverse encrusting and boring communities, and it saw a significant rise in the bioerosion of carbonate shells and hardgrounds. Especially common is the ichnogenus Gastrochaenolites. On land, large archosaurian reptiles remained dominant.

The Jurassic period was the golden age of the great sauropods, Camarasaurus, Diplodocus, Brachiosau-rus, and many others that roamed the land late during this period. They were preyed upon by large the-ropods (Ceratosaurs, Megalosaurs, and Allosaurs). During the Late Jurassic, the first birds evolved from small coelurosaur dinosaurs. Ornithischian dinosaurs were less predominant than saurischian dinosaurs, although some, such as stegosaurs and small ornitho-pods, played important roles as small and medium-to-large herbivores.

In the air, pterosaurs were common, filling many ecological roles now taken by birds. The arid, continental conditions characteristic of the Triassic steadily eased during the Jurassic period, especially at higher latitudes; the warm, humid climate allowed lush jungles to cover much of the landscape. Conifers dominated the flora, as during the Triassic; they were the most diverse group and constituted the majority of large trees. Extant conifer families that flourished during the Jurassic included the Araucariaceae,

A marine fossil: the European Jurassic geological record indicates continental submersion in tropical seas.

Cephalotaxaceae, Pinaceae, Podocarpaceae, Taxa-ceae and Taxodiaceae. The extinct Mesozoic conifer family Cheirolepidiaceae, dominated low-latitude vegetation, as did the shrubby Bennettitales. Cycads were also common, as were Ginkgos and tree ferns in the forest. Smaller ferns were probably the dominant undergrowth. Caytoniaceous seed ferns were another group of important plants during this time and are thought to have been shrub to small-tree sized. Ginkgo-like plants were particularly common in the mid- to high northern latitudes. In the Southern Hemisphere, podocarps were especially successful, while Ginkgos and Czekanowskiales were rare.

There was no polar ice during the Jurassic period, so the sea levels were higher than they are now. The climate was warm. Early in the Jurassic, the continents were jammed together into a supercontinent known as Pangaea, making much of the inland area dry and desert-like. Toward the middle of the Jurassic period, when Pangaea began to break up, there were vast flooded areas, temperate and subtropical forests, and coral reefs. The extensive water moderated the strong seasonality so that by the end of the Jurassic there was less seasonality than we have now. Many giant sauropods lived during the late Jurassic period. Conifers dominated the landscape.

There was a minor mass extinction toward the end of the Jurassic period. During this extinction, most of the stegosaurid and enormous sauropod dinosaurs died out, as did many genera of ammonoids, marine reptiles, and bivalves.

SEE ALSO: Mesozoic Era; Triassic Era.

BIBLIOGRAPHY. Tim Haines, Walking with Dinosaurs: A Natural History (Dorling Kindersley Publishing, Inc., 2000); Tjeerd van Andel, New Views on an Old Planet: A History of Global Change (Cambridge University Press, 1985).

Fernando Herrera University of California, San Diego

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