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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.
          Mesozoic era
   Triassic Jurassic Cretaceous

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:
   Upper/Late Jurassic
     Tithonian          (150.8 ± 4.0 – 145.5 ± 4.0 Ma)
     Kimmeridgian       (155.7 ± 4.0 – 150.8 ± 4.0 Ma)
     Oxfordian          (161.2 ± 4.0 – 155.7 ± 4.0 Ma)
   Middle Jurassic
     Callovian          (164.7 ± 4.0 – 161.2 ± 4.0 Ma)
     Bathonian          (167.7 ± 3.5 – 164.7 ± 4.0 Ma)
     Bajocian           (171.6 ± 3.0 – 167.7 ± 3.5 Ma)
     Aalenian           (175.6 ± 2.0 – 171.6 ± 3.0 Ma)
   Lower/Early Jurassic
     Toarcian           (183.0 ± 1.5 – 175.6 ± 2.0 Ma)
     Pliensbachian      (189.6 ± 1.5 – 183.0 ± 1.5 Ma)
     Sinemurian         (196.5 ± 1.0 – 189.6 ± 1.5 Ma)
     Hettangian         (199.6 ± 0.6 – 196.5 ± 1.0 Ma)

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.

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