The
Jurassic is a
geologic period and system that extends from
about
Ma (million years ago) to Ma, that
is, from the end of the
Triassic to the
beginning of the
Cretaceous. The Jurassic
constitutes the middle period of the
Mesozoic era, also known as the "Age of Reptiles".
The start of the period is marked by the major
Triassic–Jurassic
extinction event. However the end of the Jurassic Period did
not witness any major extinction event. The start and end of the
period are defined by carefully selected locations; the uncertainty
in dating arises from trying to date these horizons.
The chronostratigraphic term
Jurassic is directly
linked to the Swiss
Jura Mountains.
Alexander von Humboldt
(*1769, † 1859) recognised the mainly
limestone dominated mountain range of the Swiss
Jura Mountains as a separate
formation that was not at the time included in the established
stratigraphic system defined by
Abraham Gottlob Werner (* 1749, †
1817) and named it “Jurakalk” in 1795. The name “Jura” is derived
from the celtic root “jor” which was latinised into “juria”,
meaning forest (i.e. “Jura” is forest mountains).
Divisions
The Jurassic period of time is usually broken into
Early,
Middle,
and
Late Jurassic subdivisions, also
known as
Lias,
Dogger and
Malm in
Europe. The corresponding terms for the rocks are Lower, Middle,
and Upper Jurassic. The separation of the term
Jurassic into three sections goes back to
Leopold von Buch (* 1774, † 1853). The
faunal stages from youngest to oldest
are:
Paleogeography and tectonics
During the
early Jurassic period, 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. 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. 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 exposed sediments from this period are
continental, such as the alluvial deposits
of the Morrison
Formation.
The Jurassic was a time of
calcite sea
geochemistry in which low-magnesium
calcite
was the primary inorganic marine precipitate of calcium carbonate.
Carbonate hardgrounds were
thus very common, along with calcitic
ooids,
calcitic cements, and invertebrate faunas with dominantly calcitic
skeletons (Stanley and Hardie, 1998, 1999).
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
.
Africa
Early Jurassic strata are distributed in a similar fashion to Late
Triassic beds, with more common outcrops in the south and less
common fossil beds which are predominated by tracks to the north.
As the Jurassic proceeded, larger and more iconic groups of
dinosaurs like sauropods and ornithopods proliferated in Africa.
Middle Jurassic strata are neither well represented nor well
studied in Africa. Late Jurassic strata are also poorly represented
apart from the spectacular Tendeguru fauna in Tanzania. The Late
Jurassic life of Tendeguru is very similar to
that found in western
North America's
Morrison
Formation.
Fauna

Large dinosaurs were dominant during
the Jurassic Period.
Aquatic and marine
During the Jurassic, the primary vertebrates living in the seas
were
fish and marine
reptiles. The latter include
ichthyosaurs who were at the peak of their
diversity,
plesiosaurs,
pliosaurs, marine
crocodiles, of the families
Teleosauridae and
Metriorhynchidae.
In the
invertebrate world, several new
groups appeared, including
rudists (a
reef-forming variety of
bivalves) and
belemnites.
The Jurassic also had diverse encrusting and boring (sclerobiont)
communities (see Taylor & Wilson, 2003), and it saw a
significant rise in the
bioerosion of
carbonate shells and hardgrounds. Especially common is the
ichnogenus (
trace
fossil)
Gastrochaenolites.
During the Jurassic period about four or five of the twelve
clades of planktonic organisms that exist in
the fossil record either experienced a massive evolutionary
radiation or appeared for the first time.
Terrestrial
On land, large
archosaurian reptiles
remained dominant. The Jurassic was the golden age of the large
herbivorous dinosaurs known as the
sauropods—
Camarasaurus,
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 as for example
Ceratosaurus,
Megalosaurus,
Torvosaurus and
Allosaurus. All these belong to the 'lizard
hipped' or
saurischian branch of the
dinosaurs.During the Late Jurassic, the
first
birds evolved
from small
coelurosaurian 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.
The rest of the
Lissamphibia evolved in
this period, introducing the first
salamanders and
caecilians.
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.
Gymnosperms were relatively diverse during the
Jurassic period. The
Conifers in particular
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. The extinct Mesozoic conifer family
Cheirolepidiaceae dominated low
latitude vegetation, as did the shrubby
Bennettitales.
Cycads
were also common, as were
ginkgos and
Dicksoniaceous 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 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.
In the oceans modern
coralline algae
appeared for the first time.
Notes
- Hölder, H. 1964. Jura - Handbuch der stratigraphischen
Geologie, IV. Enke-Verlag, 603 pp., 158 figs, 43 tabs;
Stuttgart
- Arkell, W.J. 1956. Jurassic Geology of the World. Oliver
& Boyd, 806 pp.; Edinburgh und London.
- Pieńkowski, G.; Schudack, M.E.; Bosák, P.; Enay, R.;
Feldman-Olszewska, A.; Golonka, J.; Gutowski, J.; Herngreen,
G.F.W.; Jordan, P.; Krobicki, M.; Lathuiliere, B.; Leinfelder,
R.R.; Michalík, J.; Mönnig, E.; Noe-Nygaard, N.; Pálfy, J.; Pint,
A.; Rasser, M.W.; Reisdorf, A.G.; Schmid, D.U.; Schweigert, G.;
Surlyk, F.; Wetzel, A. & Theo E. Wong, T.E. 2008. Jurassic. In:
McCann, T. (ed.): The Geology of Central Europe. Volume 2: Mesozoic
and Cenozoic, Geological Society, pp.: 823-922;
London.
- Rollier, L. 1903. Das Schweizerische Juragebirge.
Sonderabdruck aus dem Geographischen Lexikon der Schweiz,
Verlag von Gebr. Attinger, 39 pp; Neuenburg
- Kazlev, M. Alan (2002) Palaeos website Accessed July. 22, 2008
- Late
Jurassic
- Jurassic Period
- map
- Monroe and Wicander, 607.
- Jacobs, Louis, L. (1997). "African Dinosaurs." Encyclopedia
of Dinosaurs. Edited by Phillip J. Currie and Kevin Padian.
Academic Press. p. 2-4.
- Haines, 2000.
- Behrensmeyer et al., 1992, 349.
- Behrensmeyer et al., 1992, 352
- Behrensmeyer et al., 1992, 353
References
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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) Palaeos website Accessed Jan. 8, 2006
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- Ogg, Jim; June, 2004, Overview of Global Boundary
Stratotype Sections and Points (GSSP's)
http://www.stratigraphy.org/gssp.htm Accessed April 30, 2006.
- Stanley, S.M. and Hardie, L.A. (1998). "Secular oscillations in
the carbonate mineralogy of reef-building and sediment-producing
organisms driven by tectonically forced shifts in seawater
chemistry". Palaeogeography, Palaeoclimatology, Palaeoecology 144:
3-19.
- Stanley, S.M. and Hardie, L.A. (1999). "Hypercalcification;
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External links