The Jurassic Period
Introduction: The Jurassic period boasted many walks of life- including dinosaurs and diverse vegetation. Yet the dinosaurs popularized in the Jurassic Park movie were not yet found in this time period.
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.
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.
There was a minor mass extinction toward the end of the early Jurassic period (roughly 190-183 million years ago) in which more than 80% of marine bivalve species (like many clams) and many other shallow-water species died out. The cause of this extinction is unknown, but there is some speculation (by sedimentologist Stephen P. Hesselbo et al.) that it was triggered by the release of huge methane deposits from within the Earth (these deposits formed beneath the seabed as surface algae dies and sinks to the sea floor).
Boundary: beginning approximately 210 million years ago and lasting for 70 million years of the Mesozoic Era
History: Named for the Jura Mountains on the border between France and Switzerland, where rocks of this age were first studied
Outside of Hollywood, the Jurassic is still important to us today, both because of its wealth of fossils and because of its economic importance -- the oilfields of the North Sea, for instance, are Jurassic in age.
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. Extensive Jurassic deposits can be found in the Rocky Mountains of the United States, England and western Europe, central Russia, and in many other parts of the world.
Early Jurassice (Liassic Epoch)
-age of great sea reptiles while dinosaurs rule land
Moving now from the Triassic to the Early Jurassic, we find that the dinosaurs have attained dominance, while most of the other Triassic types of animals have died out in two major Triassic extinctions - the mid-Carnian and the terminal Rhaetic. Apart from one or two early types, the dinosaurs seem to have been unaffected by these extinction events.
As the Jurassic Period opened, Italy, Greece, Turkey, and Iran were attached to the North African portion of Gondwanaland. The climate was warm and moister than during the Triassic. Reptiles were the dominant form of animal life and experienced a great adaptive radiation. In the oceans various types of ichthyosaurs and plesiosaurs evolved. In the air the pterosaurs began to diversify. On land many Triassic dinosaurs (prosauropod herbivores and coelophysid carnivores) continued, while a number of new forms (giant sauropods and armoured scelidosaurs) evolved. Under the feet of the dinosaurs rodent-like tritylodontid Therapsids co-existed with primitive shrew-like mammals and lizard-like sphenodont reptiles. Crocodiles appeared also, but they were mostly aquatic forms.
During the early Jurassic then, evolution seems to have polarised: on the one hand there were the ruling land animals, the great dinosaurs, which filled the ecological roles now taken up by medium-sized and large mammals; on the other hand the first mam-mals had appeared, and together with the tritylodont Therapsids they filled the small rodent and insectivore niche. The mam-mals were to remain small and individually insignificant - comparable to shrews, mice and rats of today - although doubtless very significant ecologically, for the 135 million years of the dinosaurs reign.
Land plants abounded in the Jurassic, but floras were different from what we see today. Although Jurassic dinosaurs are sometimes drawn with palm trees, there were no palms, or any other flowering plants, at least as we know them today, in the Jurassic. Instead, ferns, ginkgoes, bennettitaleans or "cycadeoids", and true cycads -- like the living cycad pictured at the above right -- flourished in the Jurassic. Conifers were also present, including close relatives of living redwoods, cypresses, pines, and yews. Creeping about in this foliage, no bigger than rats, were a number of early mammals.
Animals: Dinosaurs did roam the earth during this period, but unlike the movie Jurassic Park, Tyrcertops, and Tyrannesaurus Rexs had not come into existence yet. The largest dinosaurs of the time -- in fact, the largest land animals of all time -- were the gigantic sauropods, such as the famous Diplodocus (pictured at lower left), Brachiosaurus and Apatosaurus. Other herbivorous dinosaurs of the Jurassic included the plated stegosaurs. Predatory dinosaurs of the Jurassic included fearsome carnosaurs such as Allosaurus, small, fast coelurosaurs, and ceratosaurs such as Dilophosaurus. The Jurassic also saw the origination of the first birds, including the well-known Archaeopteryx, probably from coelurosaurian ancestors. 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.
But there was more to life than dinosaurs! In the seas, the fishlike ichthyosaurs were at their height, sharing the oceans with the plesiosaurs, with giant marine crocodiles, and with modern-looking sharks and rays. Also prominent in the seas were cephalopods -- relatives of the squids, nautilus, and octopi of today. Jurassic cephalopods included the ammonites, with their coiled external shells (upper left), and the belemnites, close relatives of modern squid but with heavy, calcified, bullet-shaped, partially internal shells.
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:
Ammonites (shelled cephalopods) were particularly common and diverse, forming 62 biozones. In the seas, the fishlike ichthyosaurs were at their height, sharing the oceans with the plesiosaurs, with giant marine crocodiles, and with modern-looking sharks and rays. Also prominent in the seas were cephalopods -- relatives of the squids, nautilus, and octopi of today. Jurassic cephalopods included the ammonites, with their coiled external shells (upper left. Among the plankton in the oceans, the dinoflagellates became numerous and diverse, as did the coccolithophorids (microscopic single-celled algae with an outer covering of calcareous plates).
Climate: The climate in the whole of the Earth was warm and mild.
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