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jurassic
The Jurassic (; from Jura Mountains) is a geologic period and system that extends from 201.3± 0.6 Ma (million years ago) to 145± 4 Ma; 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. Two other extinction events occurred during the period: the Late Pliensbachian/Early Toarcian event in the Early Jurassic, and the Late Tithonian event at the end; however, neither event ranks among the "Big Five" mass extinctions. The Jurassic is named after the Jura Mountains within the European Alps, where limestone strata from the period were first identified.
By the beginning of the Jurassic, the supercontinent Pangaea had begun rifting into two landmasses, Laurasia to the north and Gondwana to the south. This created more coastlines and shifted the continental climate from dry to humid, and many of the arid deserts of the Triassic were replaced by lush rainforests. On land, the fauna transitioned from the Triassic fauna, dominated by both dinosauromorph and crocodylomorph archosaurs, to one dominated by dinosaurs alone. The first birds also appeared during the Jurassic, having evolved from a branch of theropod dinosaurs. Other major events include the appearance of the earliest lizards, and the evolution of therian mammals, including primitive placentals. Crocodilians made the transition from a terrestrial to an aquatic mode of life. The oceans were inhabited by marine reptiles such as ichthyosaurs and plesiosaurs, while pterosaurs were the dominant flying vertebrates.
==Etymology==
The chronostratigraphic term "Jurassic" is directly linked to the Jura Mountains. Alexander von Humboldt recognized the mainly limestone dominated mountain range of the Jura Mountains as a separate formation that had not been included in the established stratigraphic system defined by Abraham Gottlob Werner, and he named it "Jurakalk" in 1795.〔Humboldt names the Jura limestone ("Jurakalkstein") deposits in: Alexander von Humboldt, ''Ueber die unterirdischen Gasarten und die Mittel, ihren Nachteil zu vermindern, ein Beitrag zur Physik der praktischen Bergbaukunde'' (the types of subterranean gases and means of minimizing their harm, a contribution to the physics of practical mining ) (Braunschweig, (Germany): Vieweg, 1799), (p. 39. ) From p. 39: ''" … die ausgebreitete Formation, welche zwischen dem alten Gips und neueren Sandstein liegt, und welchen ich vorläufig mit dem Nahmen Jura-Kalkstein bezeichne."'' ( … the widespread formation which lies between the old gypsum and the more recent sandstone and which I provisionally designate with the name "Jura limestone".)
He explains that he coined the name during a tour of the region in 1795 in: Alexander von Humboldt, ''Kosmos'', volume 4 (Stuttgart, (Germany): Cotta, 1858), (p. 632. ) From p. 632: ''"Ich hatte mich auf einer geognostischen Reise, die ich 1795 durch das südliche Franken, die westliche Schweiz and Ober-Italien machte, davon überzeugt, daß der Jura-Kalkstein, welchen Werner zu seinem Muschelkalk rechnete, eine eigne Formation bildete. In meiner Schrift über die unterirdischen Gasarten, welche mein Bruder Wilhelm von Humboldt 1799 während meines Aufenthalts in Südamerika herausgab, wird der Formation, die ich vorläufig mit dem Namen Jura-Kalkstein bezeichnete, zuerst (S. 39) gedacht."'' (On a geological tour that I made in 1795 through southern France, western Switzerland and upper Italy, I convinced myself that the Jura limestone, which Werner included in his shell limestone, constituted a separate formation. In my paper about subterranean types of gases, which my brother Wilhelm von Humboldt published in 1799 during my stay in South America, the formation, which I provisionally designated with the name "Jura limestone", is first conceived (p. 39).)〕〔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.〕 The name "Jura" is derived from the Celtic root "jor", which was Latinised into "juria", meaning forest (i.e., "''Jura''" is forest mountains).〔〔〔Rollier, L. 1903. Das Schweizerische Juragebirge. ''Sonderabdruck aus dem Geographischen Lexikon der Schweiz, Verlag von Gebr. Attinger,'' 39 pp; Neuenburg〕
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