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 the northern supercontinent
Laurasia 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:
rudists, a
reef-forming variety of
bivalves and
belemnites
The Jurassic had diverse encrusting and boring (sclerobiont) communities; see Taylor & Wilson (2003). It also saw a significant rise in the
bioerosion of carbonate shells and hardgrounds. Especially common is the
ichnogenus (trace fossil) ''Gastrochaenolites''.
Terrestrial
Large dinosaurs were dominant during the Jurassic Period.
On land, large
archosaurian reptiles remained dominant. The Jurassic was the golden age of the great
sauropods—
Camarasaurus,
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.
[5],
[7]
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.
★ Taylor, P.D. and Wilson, M.A., 2003. Palaeoecology and evolution of marine hard substrate communities. Earth-Science Reviews 62: 1-103.
[1]
External links
★
Examples of Jurassic Fossils
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