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 (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 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

Fossil of a young Ichthyosaur from the zoological museum of Hamburg

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.

Ichthyosaurs (Greek for "fish lizards") were giant marine reptiles that resemble a dolphin with large teeth (see convergent evolution). They were particularly abundant in the Jurassic period, until they were replaced as the top aquatic predators by plesiosaurs in the Cretaceous. Ichthyosaurs averaged 2 to 4 meters in length (although a few were smaller, and some species grew much larger), with a porpoise-like head and a long, toothed snout. They had a large tail fin and their limbs were adapted for use as steering paddles.

Plesiosaurs (IPA /ˈplisiəˌsɔɹ/) (Greek: plesios, near to + sauros, lizard) were large, carnivorous aquatic reptiles. They are somewhat fancifully said to have resembled "a turtle with a snake threaded through its body", though they lacked a shell.

They first appeared in the late Triassic period and thrived until the K-T extinction 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. They retained their ancestral two pairs of limbs, which evolved into large flippers. Plesiosaurs evolved from the earlier nothosaurs, who had a more crocodile-like body; major types of plesiosaur are primarily distinguished by head and neck size.

As a group, the plesiosaurs were the largest aquatic animals of their time, and even the smallest were about 2 m (6.5 ft) 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 dolphin-like ichthyosaurs, are known to have reached 23 m in length, and the modern whale shark (18 m), sperm whale (20 m), and especially the blue whale (30 m) are known from considerably larger specimens.

Mary Anning (1799 - 1847) was famous for her Plesiosaur discoveries at Lyme Regis in Dorset, UK. She is credited with the first Plesiosaur find (Plesiosaurus dolichodeirus), which has become the 'type fossil' (genoholotype). This region of Britain is now a World Heritage Site, dubbed the Jurassic Coast.

The first plesiosaur fossil, discovered by Mary Anning, 1821

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.

Allosaurus (pronounced /ˈæl.əʊˌsɔː.ɹəs/) meaning “different lizard”, because its vertebrae were different from those of all other dinosaurs (Greek allos = different + sauros = lizard), was a large carnivorous dinosaur with a length of up to 12 m (39 ft). It was the most common large predator in North America, 155 to 145 million years ago, in the Jurassic period.

A replica Allosaurus skeleton at a New Zealand museum.

Pterosaurs were common in the Jurassic period

Pterosaurs (/ˈte.rəʊ.sɔː(r)/, winged lizards, often referred to as pterodactyls, /ˌte.rəʊˈdæk.tɪlz/) were flying reptiles of the clade Pterosauria. They existed from the late Triassic to the end of the Cretaceous period (228 to 65 million years ago). Pterosaurs were the first 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, p.349). The extinct, Mesozoic, Conifer family Cheirolepidiaceae dominated low latitude vegetation, as did the shrubby Bennettitales (Behrensmeyer et al, 1992, p.352). 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, p.353). 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, p.352).

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.

References

ISBN 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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