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Ju·ras·sic (jʊ-răs'ĭk)  |
Of or belonging to the geologic time, rock series, or sedimentary deposits of the second period of the Mesozoic Era, in which dinosaurs continued to be the dominant land fauna and the earliest birds appeared.
n.The Jurassic Period or its system of deposits.
[French jurassique, after the JURA (MOUNTAINS).]
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The system of rocks deposited during the middle part of the Mesozoic Era, and encompassing an interval of time between about 208 and 145 million years ago, based on radiometric dating. It takes its name from the Jura Mountains, which run along the border of France and Switzerland.
The main continental masses were grouped together as the supercontinent Pangaea, with a northern component, Laurasia, separated from a southern component, Gondwana, by a major seaway, Tethys, which expanded in width eastward (see illustration). From about Middle Jurassic times onward, this supercontinent began to split up, with a narrow ocean being created between eastern North America and northwestern Africa, corresponding to the central sector of the present Atlantic Ocean. At about the same time, and continuing into the Late Jurassic, separation began between the continents that now surround the Indian Ocean, namely Africa, India, Australia, and Antarctica. As North America moved westward, it collided with a number of oceanic islands in the eastern part of the Paleo-Pacific.
Approximate distribution of land and sea in the Oxfordian stage, the first stage of the late Jurassic. Small islands are excluded, but boundaries of modern continents are included as a reference.
The climate of Jurassic times was clearly more equable than at present. A number of ferns whose living relatives cannot tolerate frost are distributed over a wide range of paleolatitudes, sometimes as far as 60° N and S. Similarly, coral reefs, which are at present confined to the tropics, occur in Jurassic strata in western and central Europe, beyond the paleotropical zone. Many other groups of organisms had wide latitudinal distribution, and there was much less endemism (restriction to a particular area) with respect to latitude than there is today. In addition, there is a lack of evidence for polar icecaps.
The vertebrate terrestrial life of the Jurassic Period was dominated by the reptiles. The dinosaurs had first appeared late in the Triassic from a thecodont stock, which also gave rise to pterosaurs and, later, birds. From small bipedal animals such as Coelophysis, there evolved huge, spectacular creatures. These include the herbivorous Apatosaurus, Brontosaurus, Brachiosaurus, Diplodocus, and Stegosaurus as well as the carnivorous, bipedal Allosaurus. See also Dinosaur.
Flying animals include the truly reptilian pterosaurs and the first animals that could be called birds as distinct from reptiles, as represented by the pigeon-sized Archaeopteryx. There were two important groups of reptiles that lived in the sea, the dolphinlike ichthyosaurs and the long-necked plesiosaurs. Both of these groups had streamlined bodies and limbs beautifully adapted to marine life. Turtles and crocodiles are also found as fossils in Jurassic deposits. See also Archaeornithes; Pterosauria.
Jurassic mammals, known mainly from their teeth alone, were small and obviously did not compete directly with the dinosaurs. The fish faunas were dominated by the holosteans, characterized by heavy rhombic scales. Their evolutionary successors, the teleosts, probably appeared shortly before the end of the period. See also Holostei.
Because they are far more abundant, the invertebrate fossil faunas of the sea are of more importance to stratigraphers and paleoecologists than are the vertebrates. By far the most useful for stratigraphic correlation are the ammonites, a group of fossil mollusks related to squids. They were swimmers that lived in the open sea, only rarely braving the fluctuating salinity and temperature of inshore waters. They are characteristically more abundant in marine shales and associated fine-grained limestones. From a solitary family that recovered from near extinction at the close of the Triassic, there radiated an enormous diversity of genera. Many of these were worldwide in distribution, but increasingly throughout the period these was a geographic differentiation into two major realms. The Boreal Realm occupied a northern region embracing the Arctic, northern Europe, and northern North America. The Tethyan Realm, with more diverse faunas, occupied the rest of the world. See also Limestone; Shale.
With regard to the plant kingdom, the Jurassic might well be called the age of gymnosperms, the nonflowering “naked seed” plants, forests of which covered much of the land. They included the conifers, gingkos, and their relatives, the cycads. Ferns and horsetails made up much of the remainder of the land flora. These and others of the Jurassic flora are still extant in much the same forms. See also Cycadales; Ginkgoales.
Jurassic source rocks in the form of organic-rich marine shale and associated rocks contain a significant proportion of the world's petroleum reserves. A familiar example is the Upper Jurassic Kimmeridge Clay of the North Sea, and its stratigraphic equivalents in western Siberia. Some of the source rocks of the greatest petroleum field of all, in the Middle East, are also of Late Jurassic age. See also Mesozoic; Petroleum geology.
| Geography Dictionary: Jurassic |
| Wikipedia: Jurassic |
| Jurassic period 199.6 - 145.5 million years ago |
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| Mean atmospheric O2 content over period duration | ca. 26 Vol %[1] (130 % of modern level) |
| Mean atmospheric CO2 content over period duration | ca. 1950 ppm[2] (7 times pre-industrial level) |
| Mean surface temperature over period duration | ca. 16.5 °C [3] (3 °C above modern level) |
The Jurassic is a geologic period and system that extends from about 199.6± 0.6 Ma (million years ago) to 145.5± 4 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.[4][5][6] The name “Jura” is derived from the celtic root “jor” which was latinised into “juria”, meaning forest (i.e. “Jura” is forest mountains).[4][5][7]
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The Jurassic period of time is usually broken into Early, Middle, and Late Jurassic subdivisions, also known as Lias, Dogger and Malm in Europe.[8] 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).[6] 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) |
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.[9] 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.[10] In contrast, the North American Jurassic record is the poorest of the Mesozoic, with few outcrops at the surface.[11] 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.[12] Important Jurassic exposures are also found in Russia, India, South America, Japan, Australasia, and the United Kingdom.
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.[13] As the Jurassic proceeded, larger and more iconic groups of dinosaurs like sauropods and ornithopods proliferated in Africa.[13] Middle Jurassic strata are neither well represented nor well studied in Africa.[13] Late Jurassic strata are also poorly represented apart from the spectacular Tendeguru fauna in Tanzania.[13] The Late Jurassic life of Tendeguru is very similar to that found in western North America's Morrison Formation.[13]
During the Jurassic, the primary vertebrates 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, 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.[8]
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 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.[14] Gymnosperms were relatively diverse during the Jurassic period.[8] 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.[15] The extinct Mesozoic conifer family Cheirolepidiaceae dominated low latitude vegetation, as did the shrubby Bennettitales.[16] Cycads were also common, as were ginkgos and Dicksoniaceous tree ferns in the forest.[8] 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.[17] Ginkgo plants were particularly common in the mid- to high northern latitudes.[8] In the Southern Hemisphere, podocarps were especially successful, while Ginkgos and Czekanowskiales were rare.[14][16]
In the oceans modern coralline algae appeared for the first time.[8]
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| Jurassic period | ||
|---|---|---|
| Lower/Early Jurassic | Middle Jurassic | Upper/Late Jurassic |
| Hettangian | Sinemurian Pliensbachian | Toarcian |
Aalenian | Bajocian Bathonian | Callovian |
Oxfordian | Kimmeridgian Tithonian |
| Preceded by Proterozoic eon | 542 Ma - Phanerozoic eon - Present | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 542 Ma - Paleozoic era - 251 Ma | 251 Ma - Mesozoic era - 65 Ma | 65 Ma - Cenozoic era - Present | ||||||||||
| Cambrian | Ordovician | Silurian | Devonian | Carboniferous | Permian | Triassic | Jurassic | Cretaceous | Paleogene | Neogene | Quaternary | |
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| Translations: Jurassic |
Dansk (Danish)
adj. - jura-, fra Jura-bjergene
n. - juratiden
Nederlands (Dutch)
betreffende de Juratijd
Français (French)
adj. - (Géol) jurassique
n. - le jurassique
Deutsch (German)
adj. - jurassisch, Jura-
n. - Jura
Ελληνική (Greek)
adj. - (γεωλ.) ιουράσιος
n. - (γεωλ.) ιουράσιος περίοδος
Português (Portuguese)
adj., -
n. - jurássico (m) (Geol.)
Русский (Russian)
юрский (период)
Español (Spanish)
adj. - jurásico
n. - jurásico
Svenska (Swedish)
adj. - jura-
n. - juraperioden
中文（简体）(Chinese (Simplified))
侏罗纪的, 侏罗系的, 侏罗纪
中文（繁體）(Chinese (Traditional))
adj. - 侏羅紀的, 侏羅系的
n. - 侏羅紀
한국어 (Korean)
adj. - 쥬라기의
n. - 쥬라기
日本語 (Japanese)
adj. - ジュラ紀の
n. - ジュラ紀
العربيه (Arabic)
(صفه) جوراسي نسبه الى احد عصور الزمن الثاني الجيولوجي (الاسم) العصر الجوراسي
עברית (Hebrew)
adj. - של תקופת היורה - התקופה השנייה של עידן המסוזואיקון
n. - מתקופת היורה (ימי הדינוזאורים) (גיאולוגיה)
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 | Dictionary. The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2007. Published by Houghton Mifflin Company. All rights reserved. Read more |
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| Sci-Tech Encyclopedia. McGraw-Hill Encyclopedia of Science and Technology. Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved. Read more |
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| Geography Dictionary. A Dictionary of Geography. Copyright © Susan Mayhew 1992, 1997, 2004. All rights reserved. Read more |
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