Jurassic
The Jurassic is a Geologic period that extends from about 135 to 195 million years before the present. As other geologic periods, the rock beds that define the start and end are well identified, but the exact dates of the start and end of the period are uncertain by 5-10 million years. The Jurassic was named by Alexandre Brongniart for extensive marine limestone exposures in the Jura Mountains in the region where Germany, France and Switzerland meet. The Jurassic follows the Triassic, is followed by the Cretaceous and constitutes the middle period of the Mesozoic Era -- the Age of Dinosaurs. The start of the Jurassic is marked by the major Triassic-Jurassic extinction event. 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 | (141-135 M.y.) |
| Kimmeridgean | (146-141 M.y.) |
| Oxfordian | (154-146 M.y.) |
| Dogger | |
| Callovian | (160-154 M.y.) |
| Batonian | (164-160 M.y.) |
| Bajocian | (170-164 M.y.) |
| Aalenian | (175-170 M.y.) |
| Lias | |
| Toarcian | (184-175 M.y.) |
| Pliensbachian | (191-184 M.y.) |
| Sinemurian | (200-191 M.y.) |
| Hettangian | (203-200 M.y.) |
During the Early Jurassic, the supercontinent of Pangea broke up into North America, Eurasia and Gondwana. But the early Atlantic and Tethyan seas 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 or ice caps. Jurassic geological record is well exposed in Western Europe, were marine sequences are found along the coast. 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:
- planctonic foraminifera and calpionelids, 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 sophisticated reptiles remained dominant. Angiosperms (flowering plants) started to appear. The first birds may have evolved during the Upper Jurassic.
See also; Geologic timescale