by Beth Spaulding
I. Setting the stage
A. What is going on in the world just before the Jurassic (Late Triassic)?
1. Position/breakup of Pangaea
Pangaea was positioned in the middle of the globe. [http://www.kaibab.org/geology/190mya.htm] By the Late Triassic, Pangaea had begun to rift apart. The waters around it were all one ocean, the Panthalassa, except for the Tethys which can be thought of as a bay of the Panthalassa, that eventually divided North and South Pangaea in the east (Chandler et al., 1992).
2. Environment: climate/sea
Occurring at the J-Tr boundary: one of the 5 most severe marine exrinctions, also with important terrestrial extinctions, though it is not well studied because of unconformities in stratigraphy of J-Tr boundary. Over 30% of all genera go extinct (Little and Benton, 1995). 92% of all Northwestern European bivalves species go extinct (Bice et al., 1992). From the 13C record, there was little affect on the "bioproductivity" of the world. The extinctions were probably due to a sharp sea level drop, followed by a sharp increase in their levels, along with the increase in anoxic bottom waters (Little and Benton, 1995). Scientists have found evidence of shocked quartz grains in Northern Tuscany indicating an extraterrestrial impact but no impact structures have been found for this boundary (Bice et al., 1992).
II. Jurassic in Europe
A. Def. of Jurassic period
The middle of the Mesozoic, inbetween Triassic and Cretaceous. The boundary between the Triassic and Jurassic was originally identified by the ammonoid near-extinction (Gilluly and Woodford, 1961). Early Jurassic: 208 Ma-187 Ma; Middle Jurassic: 187 Ma-163 Ma; Late Jurassic: 163 Ma-144 Ma (Condie and Sloan, 1997).
B. Environment: climate/sea change
In general, sea levels gradually rise throughout the Jurassic with the breakup of Pangaea. The climate was, in general, warm (almost tropical) and equable. In the Jurassic, Pangaea is divided into Tethyan and Boreal realms: Europe would have been one the border between the warmer, moister Tethyan and cooler Boreal climate.
1. Early
Around 200 Ma global sea levels rose (Ghiold, 1991). Warm shallow seas (from Tethys and Atlantic) cover England, France, Germany, and Denmark. The sea shelf did not reach Poland or most of Eastern Russia. South Russia was underneath the Tethys Sea (Gilluly and Woodford, 1961). Corresponding to high sea levels, there is no evidence of polar ice caps or any permanent ice (Chandler et al., 1992) The Jurassic global climate was hotter than today: arid in equatorial regions and humid at higher latitudes (Fastovsky and Weishaunpel, 1996).
2. Middle
Tethy's Seaway reaches almost all the way around the world at the equator (Ghiold, 1991). During Early and Middle Jurassic there is no evidence of evaporites, but there is peat and coal in Europe, which need a moist climate. Specifically, they need year round precipitation and/or very low evaporation rate (Chandler et al.,1992). The Jurassic climate was probably a more humid one because the warmer atmosphere can hold more water.
3. Late
An epicontental sea flooded much of Europe and Eastern Russia, connecting the Tethys to the Atlantic. This open seaway from North Europe across Russia brought new fauna to Europe. The large bodies of water on the continents helped to stabilize temperatures (Fastovsky and Weishaunpel, 1996). At the end of the Jurassic, the shallow seaway retreated, as evidence of seafloor deposits gradually turn to lagoon [http://www.ucmp.berkeley.edu/mesozoic/jurassic/solnhofen.html] and then continental sediments(Gilluly and Woodford, 1961). In the Late Jurassic, polar ice caps grew, the global temperature cooled and sea levels lowered around the world (Ghiold, 1991). The ancient Tethys would close as the new Atlantic ocean opened (Condie and Sloan, 1997).
C. What kind of depositional environments?
In Early Jurassic, the remnants of the Hercynian Mountains remain as stable blocks with sediment-catching basins around them. Still there was little relief in the Jurassic world: sometimes the blocks/massifs were covered with the seas and they were very low islands when they weren't covered so there is little evidence of coarse clastics (Gilluly and Woodford, 1961). Most of the depositional material is marine -clay, carbonate and shale: usually dark with carbon. However, Late Jurassic calcium carbonate deposits are called the 'White Jurassic' in Germany. The depositions of these lime muds and clays were the foundation for the great reef sponges (Ghiold, 1991).
D. What kind of tectonic activity?Orogenic activity?
On the Western shore of the Tethys, in the Alpine Region, there is evidence of extensive normal faulting. The Alpine geosyncline was making complex patterns of "uplifted and down-warped blocks" (Gilluly and Woodford, 1961). In general most of the subduction zones that had brought Pangaea together had stopped and the Jurassic was a time of sea-floor expansion.
Mediterranean Europe: crustal tension and rifting graben subsidence, submarine cracks open up, minor vulcanism (Hallam, 1975)
E. Life in the Jurassic
1. Marine E.-L.
Evidence from Southern Spain to Eastern Romania of a giant reef structure in Late Jurassic time-160Ma. Reef habitat shared by ammonites (now extinct chambered shellfish similar to nautilus), bivalves and brachiopods that attched themselves to the reef, and cyanobacteria that lived in simbiosis with the sponge reef (Ghiold, 1991). After their brush with death along the J-Tr boundary, the 2 remaining lineages of ammonoids radiated into more than 100 distinct genera (Bice et al., 1992). They are extremely useful for dating Jurassic beds. Also found in European beds are belemnoids, which are similar to modern squid. The dominant marine reptile was the plesiosaur. It shares the seas with crocodiles, turtles, and ichthyosaurs (Fastovsky and Weishaunpel, 1996).
2. Terrestrial E.-L.
a. flora
There was a similar land flora from the Late Triassic to the Early Cretaceous: dominated by cycads, ginkos, conifers and ferns (though ferns no longer 'the' dominant plant). Cycads [http://www.ucmp.berkeley.edu/seedplants/cycadophyta/cycads.html] had 6-7 ft. high barral-like trunks that produced large flowers; the trunks were topped with palm-like leaves (Gilluly and Woodford, 1961). The Jurassic also saw the first of the giant sequoia trees.
b. fauna
Early Jurassic: therapsids (including small mammals), turtles, small pterosaurs, crocodilians, 1st ornithischians and sauropods.
Middle Jurassic: sauropods and theropods found in Europe, including Megalosaurus, ankylosaurs, and stegosaurs
Late Jurassic: In Europe are the gigantic sauropods, including Brachiosaurus, Diplodocus, and Camarasaurus, along with theropods, Magalosaurus, the first birds [http://www.ucmp.berkeley.edu/diaspids/birds/archaeopteryx.html], more stegosaurs, and primitive iguanodons
III. What was to come in the Cretaceous for Europe?
Literature Cited
Bice, D. M., C. R. Newton, S. McCauley, P. W. Reiners, and C.A. McRoberts, 1992. "Shocked quartz at the triassic-jurassic boundary in italy." Science, vol. 225, pp. 443-46.
Chandler, M. A., D. Rind, and R. Ruedy, 1992. "Paleoclimate during the early jurassic: gcm simulations and the sedimentary record of paleoclimate." Geological Society of America Bulletin, vol. 104, pp. 543-59.
Condie, K. C. and R. E. Sloan, 1997. Origin and evolution of the earth: principles of historical geology. New Jersey: Prentice-Hall, Inc.
Fastovsky, D. E. and D. B. Weishaunpel, 1996. The evolution and extinction of dinosaurs. New York: Cambridge University Press.
Ghiold, Joe, 1991. "The sponges that spanned europe." New Scientist, vol. 129, pp. 58-62.
Gilluly, J. and A. O. Woodford, ed.,1961. History of the earth: an introduction to historical geology. New York: W. H. Freeman & Co.
Hallam, A., 1975. Jurassic environments. London: Cambridge University Press.
Little, C. T. S., and M. J. Benton, 1995. "Early jurassic mass extinction: a global long term event." Geology, vol. 23, pp.495-98.