Jurassic
The Jurassic period is a major unit of the geologic time scale that extends from about 200 million years ago at the end of the Triassic to 146 million years ago 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 to 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 (251 - 65 mya) | ||
|---|---|---|
| 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 (subdivisions based on fossils and used primarily by paleontologists, versus subdivisions based on rock formation and used by geologists) from youngest to oldest are:
| Upper/Late Jurassic | |
| Tithonian | 150.8 ± 4.0 – 145.5 ± 4.0 Ma (million years ago) |
| 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 (originally called Gondwanaland, this area included most of what today are the landmasses of the southern hemisphere, including Antarctica, South America, Africa, India, and Australia). In the late Jurassic, the southern continent, Gondwana, started to break up. Climates are assumed to have been 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 (study of rock layers and layering) relevance;
- rudists, a reef-forming variety of bivalves;
- belemnites; and
- brachiopods of the terebratulid and rinchonelid groups.
Ammonites (extinct, shelled cephalopods) were particularly common and diverse, forming 62 biozones.
Ichthyosaurs were particularly abundant in the Jurassic period. They were giant marine reptiles that had a porpoise-like head and a long, toothed snout, as well as a large tail fin. Ichthyosaurs averaged 2 to 4 meters in length (although a few were smaller, and some species grew much larger).
Plesiosaurs were large, aquatic reptiles as well. They first appeared in the late Triassic period and thrived until the Cretaceous-Tertiary extinction event at the end of the Cretaceous. Despite being large Mesozoic reptiles, they were not a type of dinosaur. The typical plesiosaur had a broad body and a short tail, as well a limbs in the form of flippers. As a group, the plesiosaurs were the largest aquatic animals of their time, and even the smallest were about 2 meters (6.5 feet) long. They grew to be considerably larger than the largest giant crocodiles, and were bigger than their successors, the mosasaurs. However, their predecessors as rulers of the sea, the ichthyosaurs, are known to have reached 23 meters in length. In comparison, the modern whale shark reaches 18 meters, the sperm whale 20 meters, and the blue whale 30 meters.
Terrestrial Animals
On land, large archosaurian reptiles (diapsid reptiles, including dinosaurs, as well as the pterosaurs, plesiosaurs and ichthyosaurs) remained dominant. Great, plant-eating dinosaurs, the sauropods, roamed the land, feeding on prairies of ferns and palm-like cycads and bennettitales. They were preyed upon by large carnivorous dinosaurs, the theropods, such as Ceratosaurs, Megalosaurs, and Allosaurs. Both the sauropods and theropods belong to the "lizard hipped" or saurischian branch of the dinosaurs.
Allosaurus was a large carnivorous dinosaur with a length of up to 12 meters. It was the most common large predator in North America, 155 to 145 million years ago, in the Jurassic period.
During the late Jurassic, the first birds evolved. 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.
Pterosaurs ("winged lizards"), often referred to as pterodactyls, were flying reptiles of the clade Pterosauria. They existed from the late Triassic to the end of the Cretaceous period. Pterosaurs were the first known vertebrates to evolve flight. Their wings were formed by a sophisticated membrane of skin stretching from the torso to a dramatically lengthened fourth finger. Earlier species had long, fully-toothed jaws and long tails, while later forms had a highly reduced tail, and some lacked teeth. Most pterosaurs were moderate sized, but some reached very large sizes.
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, and conifers dominated the landscape, as they had during the Triassic, and in fact were the most diverse group of trees, and constituted the greatest majority of large trees. Extant Conifer families that flourished during the Jurassic included the Araucariaceae, Cephalotaxaceae, Pinaceae, Podocarpaceae, Taxaceae, and Taxodiaceae (Behrensmeyer et al. 1992). The extinct, Mesozoic, Conifer family Cheirolepidiaceae dominated low latitude vegetation, as did the shrubby Bennettitales (Behrensmeyer et al. 1992). Cycads were also common, as were ginkgos and tree ferns in the forest; smaller ferns were probably the dominant undergrowth. Caytoniaceous seed ferns were another group of important plants during this time, and are thought to have been shrub to small-tree in size (Behrensmeyer et al. 1992). Ginkgo-like plants were particularly common in the mid- to high northern lattitudes. In the Southern Hemisphere, podocarps were especially successful (Haines 2000), while Ginkgos and Czekanowskiales were rare (Behrensmeyer et al. 1992).
ReferencesISBN links support NWE through referral fees
- Behrensmeyer, Anna K, Damuth, JD, DiMichele, WA, Potts, R Sues, HD & Wing, SL (eds.) (1992), Terrestrial Ecosystems through Time: the Evolutionary Paleoecology of Terrestrial Plants and Animals, University of Chicago Press, Chicago and London, ISBN 0226041549 (cloth), ISBN 0226041557 (paper)
- Haines, Tim (2000) Walking with Dinosaurs: A Natural History, New York: Dorling Kindersley Publishing, Inc., p. 65. ISBN 0563384492
- Kazlev, M. Alan (2002) Paleos website Accessed Jan. 8, 2006
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