Difference between revisions of "Jurassic" - New World Encyclopedia

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The Jurassic period is a major unit of the geologic time scale that extends from the end of the Triassic, about 200 million years ago, to the beginning of the Cretaceous, about 146 million years ago. 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 Jurassic is marked by the major Triassic-Jurassic extinction event, which happened just before the supercontinent Pangea started to break apart. It is estimated that over 20% of marine families and 40% of terrestrial tetrapod families went extinct, including all large Crurotarsi (a group of non-dinosaurian reptiles) and many of the large amphibians. This extinction event paved the way for the dinosaurs to assume the dominant roles in the Jurassic period.

An immense wealth of fossils have been found from the Jurassic. These provide valuable information in understanding the evolution of life on Earth. The Jurassic is important for another reason: the oilfields of the North Sea, near the United Kingdom and Norway, have much of their origin in the Jurassic.

Many people are familiar with the Jurassic period because of the popular movie Jurassic Park, based on the book by Michael Crichton. Although many of the dinosaurs depicted in that movie, such as Tyrannosaurus rex, actually belong to the subsequent Cretaceous period, the power of the media did bring a greater appreciation of, and interest in, the history of life on this planet.

The Jurassic was named by Alexander 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. These later terms originally referred to British sediments, where so many of the early fossils for this period were found. Faunal stages are also used to further delineate the period. Faunal stages are subdivisions based on fossils and used primarily by paleontologists, versus subdivisions based on rock formation and used by geologists). The faunal stages for this period, 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

Map of Pangaea

Pangaea separation animation

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

Fossil of a young Ichthyosaur from the zoological museum of Hamburg, Germany

During the Jurassic, the "highest" life forms living in the seas were fish and marine reptiles. The latter included 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 (related to the study of rock layers);
rudists, a reef-forming variety of bivalves;
belemnites (an extinct type of marine cephalopod); and
brachiopods of the terebratulid and rinchonelid groups.

The first plesiosaur fossil, discovered by Mary Anning, 1821

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, up to 23 meters in length).

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. In comparison, the modern whale shark reaches 18 meters, the sperm whale 20 meters, and the blue whale 30 meters. Their predecessors as rulers of the sea were the ichthyosaurs.

Terrestrial Animals

On land, large archosaurian reptiles (a group of diapsid reptiles that included the dinosaurs and pterosaurs, among others, including the above-mentioned Crurotarsi) 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.

A replica Allosaurus skeleton at a New Zealand museum.

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 were common in the Jurassic period

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 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 moderately 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 latitudes. In the Southern Hemisphere, podocarps were especially successful (Haines 2000), while Ginkgos and Czekanowskiales were rare (Behrensmeyer et al. 1992).

References
ISBN links support NWE through referral fees

Behrensmeyer, A. K., J. D. Damuth, W. A. DiMichele, R. Potts, H. D. Sues, and S. L. Wing. (Eds.) 1992. Terrestrial Ecosystems through Time: the Evolutionary Paleoecology of Terrestrial Plants and Animals. Chicago: University of Chicago Press.
Haines, T. 2000. Walking with Dinosaurs: A Natural History. New York: Dorling Kindersley Publishing, Inc.

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Jurassic ^history
Ichthyosaur ^history
Plesiosaur ^history
Pterosaur ^history
Allosaurus ^history
Pangea ^history
Triassic-Jurassic_extinction_event ^history

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@@ Line 69: / Line 69: @@
 [[Image:Ichthyosaur fossil.jpg|thumb|400px|Fossil of a young Ichthyosaur from the zoological museum of Hamburg, Germany]]
-During the Jurassic, the "highest" life forms living in the seas were [[fish]] and marine [[reptile]]s.  The latter include [[ichthyosaur]]s, [[plesiosaur]]s, and marine [[crocodilia|crocodiles]] of the families Teleosauridae and Metriorhynchidae.
+During the Jurassic, the "highest" life forms living in the seas were [[fish]] and marine [[reptile]]s.  The latter included [[ichthyosaur]]s, [[plesiosaur]]s, and marine [[crocodilia|crocodiles]] of the families Teleosauridae and Metriorhynchidae.
 In the [[invertebrate]] world, several new groups appeared, such as:
-* [[plankton|planktonic]] [[foraminifera]] and calpionelids, which are of great stratigraphic (study of rock layers and layering) relevance;
+* [[plankton|planktonic]] [[foraminifera]] and calpionelids, which are of great stratigraphic  relevance (related to the study of rock layers);
 * rudists, a [[reef]]-forming variety of [[bivalvia|bivalves]];
-* belemnites; and
+* belemnites (an extinct type of marine [[cephalopod]]); and
 * [[brachiopoda|brachiopods]] of the terebratulid and rinchonelid groups.
@@ Line 80: / Line 80: @@
 Ammonites (extinct, shelled [[cephalopod]]s) were particularly common and diverse, forming 62 biozones.
-[[Ichthyosaur]]s 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).
+[[Ichthyosaur]]s 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, up to 23 meters in length).
-[[Plesiosaur]]s 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 [[crocodile]]s, 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.
+[[Plesiosaur]]s 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 [[crocodile]]s, and were bigger than their successors, the mosasaurs. In comparison, the modern [[whale shark]] reaches 18 meters, the [[sperm whale]] 20 meters, and the [[blue whale]] 30 meters. Their predecessors as rulers of the sea were the ichthyosaurs.
 == 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 [[fern]]s 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 [[dinosaur]]s.
+On land, large archosaurian reptiles (a group of diapsid reptiles that included the [[dinosaur]]s and [[pterosaur]]s, among others, including the above-mentioned Crurotarsi) remained dominant. Great, plant-eating dinosaurs, the sauropods, roamed the land, feeding on prairies of [[fern]]s 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 [[dinosaur]]s.
 [[Image:Allosaurus1.jpg|thumb|210px|left|A replica Allosaurus skeleton at a New Zealand museum.]]
-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.
+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 [[bird]]s [[Evolution|evolved]]. Ornithischian dinosaurs were less predominant than saurischian dinosaurs, although some like [[stegosaur]]s and small ornithopods played important roles as small and medium-to-large (but not sauropod large) herbivores.  In the air, [[pterosaur]]s were common, filling many ecological roles now taken by birds.
 [[Image:anhanguera-piscator_jconway.jpg|thumb|right|250|Pterosaurs were common in the Jurassic period]]
-Pterosaurs ("winged lizards"), often referred to as ''pterodactyls'', were flying [[reptile]]s of the clade '''Pterosauria'''. They existed from the late [[Triassic]] to the end of the [[Cretaceous]] period. Pterosaurs were the first known [[vertebrate]]s 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.
+Pterosaurs ("winged lizards"), often referred to as ''pterodactyls'', were flying [[reptile]]s of the clade ''Pterosauria''. They existed from the late [[Triassic]] to the end of the [[Cretaceous]] period. Pterosaurs were the first known [[vertebrate]]s to evolve flight. Their wings were formed by a 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 moderately 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). [[Angiosperms | 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 [[ginkgo]]s 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).
+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). [[Angiosperms | 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 [[tree]]s, 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 [[ginkgo]]s 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 latitudes.  In the Southern Hemisphere, podocarps were especially successful (Haines 2000), while Ginkgos and Czekanowskiales were rare (Behrensmeyer et al. 1992).