Jurassic
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.
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:
During the early Jurassic, the
supercontinent Pangea broke up into
North America,
Eurasia and
Gondwana. Still, the early
Atlantic and
Tethyan Oceans were 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.(see [
1]) 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 and extending into the Cretaceous, marking the
Nevadan orogeny.(Monroe and Wicander, 607) Important Jurassic exposures are also found in
Russia,
India,
South America,
Japan,
Australasia, and the
United Kingdom.
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) were particularly common and diverse, forming 62
biozones.
On land, large
archosaurian reptiles remained dominant. Great plant-eating dinosaurs (
sauropods) roamed the land, 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, filling many ecological roles now taken by birds.
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 (Haines 2000).
Flowering plants had not evolved yet, and
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 (Behrensmeyer
et al, 1992, p.349). The extinct Mesozoic Conifer family
Cheirolepidiaceae dominated low latitude vegetation, as did the shrubby
Bennettitales (Behrensmeyer
et al., 1992, p.352).
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 (Behrensmeyer
et al., 1992, p.353). Ginkgo-like plants were particularly common in the mid- to high northern lattitudes. In the Southern Hemisphere,
podocarps were especially successful (Haines 2000), while Ginkgos and
Czekanowskiales were rare (Behrensmeyer
et al., 1992, p.352).
*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 actually from the
Cretaceous period.
*There is a
hip hop group called
Jurassic 5.
*
Behrensmeyer, Anna K,
Damuth, JD,
DiMichele, WA,
Potts, R Sues, HD &
Wing, SL (eds.) (1992),
Terrestrial Ecosystems through Time: the Evolutionary Paleoecology of Terrestrial Plants and Animals,
University of Chicago Press, Chicago and London, ISBN 0226041549 (cloth), ISBN 0226041557 (paper)
* Haines, Tim (2000)
Walking with Dinosaurs: A Natural History, New York: Dorling Kindersley Publishing, Inc., p. 65. ISBN 0563384492
* Kazlev, M. Alan (2002)
Paleos website Accessed Jan. 8, 2006
*Monroe, James S., and Reed Wicander. (1997)
The Changing Earth: Exploring Geology and Evolution, 2nd ed. Belmont: West Publishing Company, 1997. ISBN 0314095772
* Ogg, Jim; June, 2004,
Overview of Global Boundary Stratotype Sections and Points (GSSP's) http://www.stratigraphy.org/gssp.htm Accessed April 30, 2006.
