Jurassic

2007 Schools Wikipedia Selection. Related subjects: Geology and geophysics

The Jurassic Period is a major unit of the geologic timescale that extends from about 200 Ma (million years ago), at the end of the Triassic to 146 Ma, at the beginning of the Cretaceous. As with other geologic periods, the rock beds that define the start and end of the period are well identified, but the exact dates are uncertain by 5 - 10 million years. The Jurassic constitutes the middle period 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.

The Jurassic was named by Alexandre Brogniart for the extensive marine limestone exposures of the Jura Mountains, in the region where Germany, France and Switzerland meet.

Divisions

The Jurassic period of time is usually broken into Early, Middle, and Late subdivisions, also known as Lias, Dogger and Malm. The corresponding terms for the rocks are Lower, Middle, and Upper Jurassic. The faunal stages from youngest to oldest are:

Paleogeography

During the early Jurassic, the supercontinent Pangea broke up into North America, Eurasia and Gondwana. Still, the early Atlantic Ocean was relatively narrow. In the Late Jurassic, the southern continent, Gondwana, started to break up , and as the Tethys closed, the Neotethys 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 is good in western Europe, where extensive marine sequences are found along the coasts, including the famous Jurassic Coast World Heritage Site. The strata of this period also feature the renowned lagerstätten of Holzmaden and Solnhofen. In contrast, the North American Jurassic record is the poorest of the Mesozoic, 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 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 Russia, India, South America, Japan, Australasia, and the United Kingdom.

Aquatic and Marine Animals

During the Jurassic, the 'highest' life forms living in the seas were fish and marine reptiles. The latter include ichthyosaurs, plesiosaurs and marine crocodiles, of the families Teleosauridae and Metriorhynchidae.

In the invertebrate world, several new groups appeared, such as:

• planktonic foraminifera and calpionelids, which are of great stratigraphic relevance;
• rudists, a reef-forming variety of bivalves;
• belemnites; and
• brachiopods of the terebratulid and rinchonelid groups.

Ammonites (shelled cephalopods) are particularly common and diverse, forming 62 biozones.

Terrestrial Animals

On land, large archosaurian reptiles remained dominant. Great plant-eating dinosaurs ( sauropods) roamed the land late in the period, feeding on prairies of ferns and palm-like cycads and bennettitales. They were preyed upon by large theropods (Ceratosaurs, Megalosaurs, and Allosaurs). All these belong to the 'lizard hipped' or saurischian branch of the dinosaurs.

During the Late Jurassic the first birds evolved from small coelurosaur dinosaurs. Ornithischian dinosaurs were less predominant than saurischian dinosaurs, although some like stegosaurs and small ornithopods played important roles as small and medium-to-large (but not sauropod-sized) herbivores. In the air, pterosaurs were common; they ruled the skies, filling many ecological roles now taken by birds.

Plants

The arid conditions that had characterized much 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 landscape, as they had during the Triassic. In fact they were the most diverse group of trees and constituted the greatest majority of large trees. Extant Conifer families that flourished during the Jurassic included the Araucariaceae, Cephalotaxaceae, Pinaceae, Podocarpaceae, Taxaceae 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,(Haines 2000), while Ginkgos and Czekanowskiales were rare.,

Popular culture

• The name of the novel and movie Jurassic Park referred to the Jurassic period, although many of the creatures featured in the novel and movie are from the Cretaceous period.
• There is a hip hop group called Jurassic 5.