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The Jurassic period is a major unit of the geologic timescale that extends from about 195 million years BP at the end of the Triassic to 135 million years BP 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 Brongniart for the extensive marine limestone exposures of the Jura Mountains in the region where Germany, France and Switzerland meet.
Divisions
The Jurassic is usually broken into lower, middle, and upper subdivisions, also known as Lias, Dogger and Malm. The Faunal stages from youngest to oldest are: | Malm | | | Tithonian | (135–141 M.y.) | | Kimmeridgean | (141–146 M.y.) | | Oxfordian | (146–154 M.y.) | | Dogger | | | Callovian | (154–160 M.y.) | | Bathonian | (160–164 M.y.) | | Bajocian | (164–170 M.y.) | | Aalenian | (170–175 M.y.) | | Lias | | | Toarcian | (175–184 M.y.) | | Pliensbachian | (184–191 M.y.) | | Sinemurian | (191–200 M.y.) | | Hettangian | (200–203 M.y.) |
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. Climates were warm with no evidence of glaciation. As in the Triassic, apparently there was no land near either pole, and no extensive ice caps existed. The geological record of the Jurassic is well exposed in western Europe, were marine sequences are found along the coasts. A shallow sea (epicontinental sea) was present in parts of the northern plains of the United States and Canada. Most Jurassic exposures in North America are continental. 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. In the invertebrate world, several new groups appeared, such as:
Ammonites (shelled cephalopods) were particularly common and diverse, forming 62 different biozones. On land, large sophisticated reptiles remained dominant. Great plant-eating dinosaurs roamed the land, feeding on lush growths of ferns and palm-like cycads and bennettitaleans. Angiosperms (flowering plants) started to appear. The first birds may have evolved during the upper Jurassic, however this point is disputed. Some estimates of the age of the Liaoning lagerstätte go as far back as the upper Jurassic, and the birds containted there are definitely not "primitive").
See Also Though the movie Jurassic Park brought the word "Jurassic" to household usage, many of the creatures featured in that film would more likely be found in the Cretaceous period.
For the hip hop group see Jurassic 5.
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