Jurassic

The Jurassic Period is a major unit of the geologic timescale that extends from about 199.6 ± 0.6 Ma (million years ago) to 145.4 ± 4.0 Ma, the end of the Triassic to 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 Pangaea broke up into North America, Eurasia 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.^[1] The Tethys Sea closed, and 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 indicate a time when much of the continent was submerged under shallow tropical seas; famous locales include the Jurassic Coast World Heritage Site and the renowned late Jurassic lagerstätten of Holzmaden and Solnhofen.^[2] In contrast, the North American Jurassic record is the poorest of the Mesozoic, with few outcrops at the surface.^[3] 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.^[4] Important Jurassic exposures are also found in Russia, India, South America, Japan, Australasia, and the United Kingdom.

Fauna

Aquatic and marine

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

On land, large archosaurian reptiles remained dominant. The Jurassic was the golden age of the great sauropods—Apatosaurus, Diplodocus,Brachiosaurus, and many others—that roamed the land late in the period; their mainstays were either the prairies of ferns, palm-like cycads and bennettitales, or the higher coniferous growth, according to their adaptations. 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.

Flora

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.^[5] 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, Cephalotaxaceae, Pinaceae, Podocarpaceae, Taxaceae and Taxodiaceae.^[6] The extinct Mesozoic conifer family Cheirolepidiaceae dominated low latitude vegetation, as did the shrubby Bennettitales.^[7] 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.^[8] 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.^[9],^[10]

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, such as Tyrannosaurus and Triceratops.
• There is a hip hop group called Jurassic 5.
• The Museum of Jurassic Technology is located in Los Angeles, California.

Notes:

1. ^ http://www.scotese.com/late1.htm
2. ^ http://www.urweltmuseum.de/Englisch/museum_eng/Geologie_eng/Tektonik_eng.htm
3. ^ http://www.nationalatlas.gov/articles/geology/legend/ages/jurassic.html|map]
4. ^ Monroe and Wicander, 607.
5. ^ Haines, 2000.
6. ^ Behrensmeyer et al, 1992, 349.
7. ^ Behrensmeyer et al., 1992, 352
8. ^ Behrensmeyer et al., 1992, 353
9. ^ Haines, 2000.
10. ^ Behrensmeyer et al., 1992, 352

References

• Behrensmeyer, Anna K., Damuth, J.D., DiMichele, W.A., Potts, R., Sues, H.D. & Wing, S.L. (eds.) (1992), Terrestrial Ecosystems through Time: the Evolutionary Paleoecology of Terrestrial Plants and Animals, University of Chicago Press, Chicago and London, ISBN 0-226-04154-9 (cloth), ISBN 0-226-04155-7 (paper)
• Haines, Tim (2000) Walking with Dinosaurs: A Natural History, New York: Dorling Kindersley Publishing, Inc., p. 65. ISBN 0-563-38449-2
• Kazlev, M. Alan (2002) Paleos website Accessed Jan. 8, 2006
• Mader, Sylvia (2004) Biology, eighth edition
• Monroe, James S., and Reed Wicander. (1997) The Changing Earth: Exploring Geology and Evolution, 2nd ed. Belmont: West Publishing Company, 1997. ISBN 0-314-09577-2
• Ogg, Jim; June, 2004, Overview of Global Boundary Stratotype Sections and Points (GSSP's) http://www.stratigraphy.org/gssp.htm Accessed April 30, 2006.

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