Jurassic
The Jurassic period is a major unit of the geologic timescale that extends from about 200 Ma (million years ago) at the end of the Triassic to 146 Ma 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.
| Mesozoic era | ||
|---|---|---|
| Triassic | Jurassic | Cretaceous |
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/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) |
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 and as the Tethys closed the Neotethys basin appeared. 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, where marine sequences are found along the coasts. A shallow sea ( epicontinental sea) called the Sundance 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.
Aquatic and Marine Animals
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 biozones.
Terrestrial Animals
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 bennettitales. They were preyed upon by large theropods (Ceratosaurs, Megalosaurs, and Allosaurs). All these belong to the "lizard hipped" or saurischian branch of the dinosaurs.
During the late Jurassic the first birds evolved from small coelurosaur 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 large) herbivores. In the air, pterosaurs were common, filling many ecological roles now taken by birds.
Plants
The arid conditions that had characterized much of the Triassic steadily eased during the following period, especially at higher latitudes; the warm, humid climate allowed lush jungles to cover much of the landscape (Haines 2000). Flowering plants had not evolved yet, but conifers were common, and in fact were the most diverse group of trees, as were palms Kazlev (2002). Cycads were abundant, as were ginkgos and tree ferns in the forest; smaller ferns were probably the dominant undergrowth Kazlev (2002). In the Southern Hemisphere, podocarps were especially successful (Haines 2000).
Popular culture
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.