Jurassic
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The Jurassic period is a major unit of the geologic timescale that extends from about 200 million years BP at the end of the Triassic to 146 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 Brogniart 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 (or Early, Middle and Late) subdivisions, also known as Lias, Dogger and Malm. The faunal stages from youngest to oldest are:
| Upper Jurassic | |
| Tithonian | (150.8 ± 4.0 – 145.5 ± 4.0 MYA) |
| Kimmeridgean | (155.7 ± 4.0 – 150.8 ± 4.0 MYA) |
| Oxfordian | (161.2 ± 4.0 – 155.7 ± 4.0 MYA) |
| Middle Jurassic | |
| Callovian | (164.7 ± 4.0 – 161.2 ± 4.0 MYA) |
| Bathonian | (167.7 ± 3.5 – 164.7 ± 4.0 MYA) |
| Bajocian | (171.6 ± 3.0 – 167.7 ± 3.5 MYA) |
| Aalenian | (175.6 ± 2.0 – 171.6 ± 3.0 MYA) |
| Lower Jurassic | |
| Toarcian | (183.0 ± 1.5 – 175.6 ± 2.0 MYA) |
| Pliensbachian | (189.6 ± 1.5 – 183.0 ± 1.5 MYA) |
| Sinemurian | (196.5 ± 1.0 – 189.6 ± 1.5 MYA) |
| Hettangian | (199.6 ± 0.6 – 196.5 ± 1.0 MYA) |
Paleogeography
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.
Animals and Plants
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 different 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 bennettitaleans. They were preyed upon by large theropods (Ceratosaurs, Megalosaurs, and Allosaurs). During the late Jurassic the first birds evolved from small coelurosaur dinosaurs. All these belong to the "lizard hipped" or saurischian branch of the dinosaurs. In contrast, Ornithischian dinosaurs were less predominant, although some like stegosaurs and small ornithopods played important roles as small-medium- to large (but not sauropod large) herbivores. In the air, pterosaurs were common, filling many ecological roles now taken by birds.