Jurassic The splendidly fossiliferous limestones and shales of the Jura Mountains of Switzerland attracted the attention of the early scientists of Europe long before their true nature was realized. They eventually drew the attention of the traveller and scientist, Alexander von Humboldt (1769–1859) who in 1795 had written about them, using the term Jura-Kalkstein. Somewhat later, when Leopold von Buch came to study them in the 1830s, their correlation with similar fossiliferous beds in England and Germany became apparent to him and he used Humboldt's term in establishing a formal system of rocks for all three areas (1839). The name was immediately used by the many amateur and professional geologists who then were beginning to make maps of the fossilerous formations in the various parts of Europe. The ammonite fossils were everywhere an immediate clue to the age and correlation of the rocks of the Jurassic system. They are distinct from those of the marine Triassic rocks beneath and from the ammonites of the Cretaceous system above.

The Jurassic is the second of the three Mesozoic systems, and was formed over a span of about 70 million years prior to the commencement of the Cretaceous period (135 Ma). The system is divided into 11 stages and over 70 ammonite zones in Britain. Virtually all the stages are named after localities in western Europe where the system is extensively developed in neritic (shallow-water) facies. The German palaeontologist Albert Oppel realized in the mid-nineteenth century that ammonites give by far the best means of correlation within the Jurassic. They are widespread and evolved rapidly; only within a few nearshore or brackish-water facies are they rare or absent. Oppel's detailed zonal scheme for the Jurassic has provided the basis for our modern zonation. Moreover, subzones can be defined within many of the Jurassic ammonite zones, and even smaller biostratigraphic units (horizons) characterized by the presence of a short-lived species have been identified. There are some 44 of these in the Middle Jurassic Aalenian and Bajocian Stages in western Europe. This is biostratigraphy on a very detailed scale. Where alternative zonal schemes have been erected—using foraminifera or coccolith microfossils, for example—the species used have longer time ranges than the ammonites. None of the Triassic superfamilies of ammonites lived on into the Jurassic, but three new superfamilies appeared in the Early Jurassic. Significant extinction events of ammonites occurred in the late Early Jurassic, at the Middle–Late Jurassic boundary, and at the end of the period. Such was the palaeogeography of the Jurassic that the biota was distributed across a number of palaeoecological provinces; and this provinciality poses a number of problems. This is apparent in the uppermost stage, known variously as the Tithonian, Portlandian, and Volgian in different parts of the Eurasia. They are not exactly equivalent, and the Volgian has a boreal fauna, different from the Tethyan (Alpine–Himalayan) faunas of the other two. In the absence of ammonites, the dinoflagellates may provide marine correlation, while spores and pollens serve best in non-marine facies.

Many stratigraphers have regarded this system in western Europe as reflecting a full cycle of marine deposition, beginning with a marine transgression and ending with a sharp regression. Detailed study, however, has shown that this is an oversimplification: several important transgressions follow on from phases of uplift and erosion, despite the then relative stability of much of the western European continental area.

The palaeogeography of the Jurassic period was one of change, with the break-up and dispersal of the rifted fragments of Pangaea now well under way (Fig. 1). The two principal continental masses, Laurasia and Gondwanaland, became increasingly separated by the equatorial Tethyan ocean. During the latter half of the period, north-western Africa parted from North America to create a narrow ocean, the forerunner of the Atlantic. Somewhat later the separation of South America, Africa, and Antarctica began, while ocean floor spreading in the proto-Pacific Ocean began to move small land masses on to the margins of Asia and the Americas. Plate-tectonic motions thus appear in the Jurassic period to have been more important in bringing about geographical change, with accompanying rise in sea level and climatic amelioration, than they had been in the Triassic period. Volcanic activity took place on an enormous scale in various parts of the world. Dyke-swarms were intruded in many regions of Africa. The lavas of the Karroo in southern Africa covered perhaps 2 000 000 km² to a thickness approaching 9 km. Lavas are also present in a zone running across the centre of Antarctica.

One of the most tectonically active regions of the Mesozoic world was the great belt of island arcs, deep troughs, and volcanoes stretching along the western margin of the Americas from Alaska to Panama and southwards to southern Chile. There was almost continuous crustal unrest and volcanism here, and at depth beneath the newly rising mountains great batholiths of granite and granodiorite were intruded. Volcanic rocks of enormous thickness and deep-seated igneous intrusions from this period have created much of the geology of the Peruvian Andes. Subduction of the Pacific crustal plates beneath the margins of the North American and South American plates was responsible, and the process has continued ever since.

It was a time of slow, impersistent but inevitable rise in sea level, with up to a quarter of the present continental area flooded in Late Jurassic time. The period began with sea level at a relatively low stand at around 14 per cent of the continental area under water and rather less at the end. Many geologists see this period as exhibiting a great eustatic cycle within which many smaller cycles developed. Only towards the very end of the period did sea level begin to decline rather sharply. This Jurassic inundation was very extensive in Laurasia, but most of North America and Gondwanaland was not greatly affected. Nevertheless, a narrow sea spread down the easthern flank of the present Rockies during the mid-part of the period. Much of eastern Asia persisted as an upland area. Palaeomagnetic polarity was very mixed throughout the Jurassic, but with an interval of relatively steady normal polarity in mid-period.

The climate seems to have been warm and equable; with no polar icecaps and with an equatorial ocean, conditions existed to give tropical climates well up into the present north temperate latitudes. The continental interiors, distant from the sea, were generally low-lying and arid.

Marine life was prolific with a widespread development of both calcareous benthonic organisms and vigorous free-moving forms. Reefs of corals, algae, bryozoa, and sponges are known, while the bivalvia were the almost universally dominant stock of the benthos elswhere. Gastropods, belemnites, echinoids, foraminifera, and ostracods were important in many deposits. A complex picture of developing faunal provinces emerges from studies of the Jurassic shallow-water facies. It appears to depend upon many factors, including climate and water circulation. Many areas of very strong salinity have been demonstrated. Open-water planktonic and nektonic faunas were dominated by the cephalopods. The ammonites were by far the most prolific, with many fast-evolving lineages. They have proved to be most useful in biostratigraphy because of their robust preservation, rapid evolution, and widespread distribution. This serves to illustrate the remarkable change in cephalopod fortunes following the late Triassic extinction, when only one family survived into the Jurassic. Marine vertebrates included the large dolphin-like ichthyosaurs and the long-necked plesiosaurs, marine crocodiles, turtles, and other reptiles. So high was local organic productivity during Jurassic times that many sedimentary basins became rich in petroleum hydrocarbons.

On land, there was a rich flora in the more rainy regions to provide swamp and forest coverage. Gymnosperms, including conifers, gingkoes, and cycads, were very numerous, as were ferns and horsetails. From the Jurassic forests are derived extensive coal deposits, some of the seams being 5 m or more in thickness. They are largely confined to the eastern parts of Laurasia and Gondwanaland, where a climate for long-continuing forest growth persisted into the Cretaceous. Palaeobotanical studies suggest that the temperature gradient from low to high latitudes was significantly less than today in the northern hemisphere.

Tetrapod populations were overwhelmingly of reptiles. The Triassic thecodonts had given rise to the dinosaurs and pterosaurs in the late Triassic, and these now gave rise to an extraordinary range of adaptations to new habitats. Bipedal and quadrupedal herbivors achieved considerable size, while predatory types also are well represented by active bipedal forms. By late Jurassic time the pigeon-sized Archaeopteryx had evolved and true birds were soon to follow. The mammals remained small and relatively subordinate, but the insectivors and omnivors were undoubtedly diverse and numerous. Among the fishes the holosteans, with their thick enamelled scales, were the most prolific. Some were of very large size. The sharks included large species that preyed upon the schools of cephalopods, and shell-feeding forms that snatched large bivalves from the sea floor.

The natural resources of the Jurassic system are very varied and extensive. They include all manner of construction materials, mineral ores, and non-metallics associated with igneous and sedimentary formations, fossil fuels, and other strategic materials.

D. L. Dineley

Bibliography

Arkell, W. J. (1956) Jurassic geology of the world. Oliver and Boyd, Edinburgh.
Hallam, A. (1975) Jurassic environments. Cambridge University Press.
Hallam, A. (1984) Continental humid and arid zones in the Jurassic and Cretaceous. Palaegeography, Palaeoclimatology, Palaeoecology, 46, 195–223.
Sellwood, B. W. (1978) Jurassic. In McKerrow, W. S. (ed.) The ecology of fossils. Duckworth, London.

Jurassic

The Jurassic period is the second of the three divisions of the Mesozoic era, "The Age of Reptiles." The Jurassic lasted for 64 million years, from about 208 to 144 million years ago. The period is named for rock strata found in the Jura Mountains on the border between Switzerland and France.

During the Jurassic, the supercontinent Pangaea began to break apart. This created two landmasses, a northern mass called Laurasia (North America, Europe, and Asia) and a southern mass called Gondwanaland (South America, Africa, Australia, Antarctica, and India). During the early Jurassic, North America separated from Africa and South America and moved northward, but still remained connected to Europe. By the late Jurassic, the North Atlantic was just beginning to appear between Europe and North America.

Widespread deposits of sand in western North America indicate that the region experienced a desert climate during the early Jurassic. Coral reefs and the remains of temperate and subtropical forests around the world provide evidence that the climate became moister and milder later in the period. Europe was covered with shallow seas throughout the Jurassic.

Jurassic vegetation consisted mainly of seed ferns, cycads, horsetails, conifers, and gingkoes. The Jurassic is sometimes called the "Age of Cycads" because of the variety and diversity of these seed-bearing, palmlike plants. Some cycads grew to be tall as trees; other forms were short and squat.

In the marine world, the great success story was that of the ammonites —the coiled, shelled relatives of modern squid. At the end of the Triassic (the period just before the Jurassic), nearly 47 percent of marine species went extinct, indicating a drastic rapid deterioration of the environment that results in a crisis for certain species and is known as an extinction event. Extinction events allow some species to adapt to different environmental conditions and fill new niches. This is known as adaptive radiation. Although only one family of ammonites survived an extinction event at the end of the Triassic, this family radiated into an astonishing array of forms, some of which attained sizes of 2 meters (6 feet) or more.

The Jurassic period is known for an increase in the numbers and diversity of dinosaurs. At the beginning of the period, dinosaurs such as the bipedal and carnivorous theropods were small and lightly boned, feeding on insects or other small dinosaurs. By the close of the period, massive predators like Allosaurus and Ceratosaurus had appeared. These dinosaurs had heavy bodies, powerful hind legs, front limbs used for grasping and holding prey, and long, sharp teeth for spearing and stabbing. The largest of all dinosaurs, the plant-eating sauropods, also developed during the Jurassic. The sauropods include Apatosaurus (formerly called Brontosaurus), Brachiosaurus, Diplodocus, Seismosaurus, and Suprasaurus. These animals were quadrupeds, with pillarlike legs (like the legs of an elephant) that supported their enormous body weight, which was often 18 metric tons (20 tons) or more. The large size of the sauropods may have helped them maintain a consistent body temperature. The Stegosaurus is known for a distinctive row of heavy, triangular, bony plates, known as scutes, which were arranged along its back. Paleontologists (scientists who study dinosaurs) believe these plates helped the Stegosaurus regulate its body temperature and protected it from being eaten. Several sharp, bony spikes on the end of the tail of Stegosaurus probably served as a weapon against attack.

The debate continues as to whether birds most likely evolved from small, bipedal dinosaurs or other ground-dwelling reptilian ancestors. Archaeopteryx is one of the earliest undisputed bird fossils. It exhibits features of both dinosaurs and birds, including a long, bony tail; small, sharp teeth; feathers; and a "wishbone" that allowed for the attachment of flight muscles.

Mammals continued to diversify during the Jurassic, but remained small and nocturnal, possibly to avoid competition with the dinosaurs. These early mammals were almost all herbivores, insectivores, and frugivores (fruit eaters).

see also Geological Time Scale.

Leslie Hutchinson

Bibliography

Lane, Gary, A., and William Ausich. Life of the Past, 4th ed. Upper Saddle River, NJ: Prentice Hall, 1999.

Jurassic Period

In geologic time , the Jurassic Period—the middle of three geologic periods in the Mesozoic Era—spans the time from roughly 206–208 million years ago (mya) to approximately 146 mya.

The Jurassic Period contains three geologic epochs. The earliest epoch, the Lias Epoch, ranges from the start of the Jurassic Period to approximately 180 mya. The Lias Epoch is further subdivided into (from earliest to most recent) Hettangian, Sinemurian, Pliensbachian, and Toarcian stages. The middle epoch, the Dogger Epoch, ranges from 180 mya to 159 mya and is further subdivided into (from earliest to most recent) Aalenian, Bajocian, Bathonian, and Callovian stages. The latest epoch (most recent), the Maim Epoch, ranges from 159 mya to 144 mya and is further subdivided into (from earliest to most recent) Oxfordian, Kimmeridgian, and Tithonian stages.

During the Jurassic Period, the Pangaean supercontinent broke into continents recognizable as the modern continents. At the start of Jurassic Period, Pangaea spanned Earth's equatorial regions and separated the Panthalassic Ocean and the Tethys Ocean. Driven by plate tectonics during the Jurassic Period, the North American and European continents diverged, and the earliest form of the Atlantic Ocean flooded the spreading sea floor basin between the emerging continents. By mid-Jurassic Period, although still united along a broad region, what would become the South American and African Plates and continents became distinguishable in a form similar to the modern continents.

By the end of the Jurassic Period, North America and South America became separated by a confluence of the Pacific Ocean and Atlantic Ocean. Extensive flooding submerged much of what are now the eastern and middle portions of the United States.

By the end of the Jurassic Period, water separated South America from Africa , and the Australian and Antarctic continents were clearly articulated. The Antarctic continent began a slow southward migration toward the south polar region.

The Jurassic Period (in popular culture widely recognized as the "Age of the Dinosaurs") was named for the Jura Mountains on the Swiss-French border, an area where the classic formations were first identified and studied.

Large meteor impacts occurred at the start and end of the Jurassic Period (and later intensified during the subsequent Cretaceous Period ). During the Jurassic Period itself, there is evidence of only one major impact—the Puchezh impact in Russia. The Manicouagan impact in crust now near Quebec, Canada, dates to the late Triassic Period just before the start of the Jurassic Period. A trio of impacts in areas now located in South Africa, the Barents Sea, and Australia occurred near the end of the Jurassic Period and start of the Cretaceous Period.

Although humans and dinosaurs never co-existed—in fact they are separated by approximately 63 million yeas of evolutionary time—the Jurassic Period's wealth of fossils have long stirred human imagination about life on Earth during that time. The abundant life of the Jurassic Period also left a legacy of organic remains that today provide an economically important source of fossil fuels . Many prominent oil-fields date to the Jurassic Period (e.g., the North Sea fields).

See also Archean; Cambrian Period; Cenozoic Era; Dating methods; Devonian Period; Eocene Epoch; Evolution, evidence of; Fossil record; Fossils and fossilization; Geologic time; Historical geology; Holocene Epoch; Marine transgression and marine recession; Miocene Epoch; Mississippian Period; Oligocene Epoch; Ordovician Period; Paleocene Epoch; Paleozoic Era; Pennsylvanian Period; Phanerozoic Eon; Pleistocene Epoch; Pliocene Epoch; Precambrian; Proterozoic Era; Quaternary Period; Silurian Period; Supercontinents; Tertiary Period

Jurassic The second geological period of the Mesozoic era. It followed the Triassic, which ended about 213 million years ago, and extended until the beginning of the Cretaceous period, about 144 million years ago. It was named in 1829 by A. Brongniart after the Jura Mountains on the borders of France and Switzerland. Jurassic rocks include clays and limestones in which fossil flora and fauna are abundant. Plants included ferns, cycads, ginkgos, rushes, and conifers. Important invertebrates included ammonites (on which the Jurassic is zoned), corals, brachiopods, bivalves, and echinoids. Reptiles dominated the vertebrates and the first flying reptiles – the pterosaurs – appeared. The first primitive bird, Archaeopteryx, also made its appearance.

Jurassic the second period of the Mesozoic era, between the Triassic and Cretaceous periods. The Jurassic lasted from about 208 to 146 million years ago. Large reptiles, including the largest known dinosaurs, were dominant on both land and sea. Ammonites were abundant, and the first birds (including Archaeopteryx) appeared.

The name comes from French jurassique, from the Jura mountains on the border of France and Switzerland.
Jurassic Park the title of a thriller (1990) by Michael Crichton and the Spielberg film based on it, in which dinosaurs were cloned from fossil DNA to stock a theme park; the result was carnage as the systems for control and safety failed. Jurassic Park is now referred to as a type of environment where savagery prevails.

Jurassic One of the three Mesozoic periods: it lasted from 208 to 145.6 Ma, following the Triassic and preceding the Cretaceous. The Jurassic Period is subdivided into 11 stages (Hettangian, Sinemurian, Pliensbachian, Toarcian, Aalenian, Bajocian, Bathonian, Callovian, Oxfordian, Kimmeridgian, and Tithonian), with clays, calcareous sandstones, and limestones being the most common rock types. Brachiopods, bivalves, and ammonites are abundant fossils, along with many other invertebrate stocks. Reptiles flourished on land and in the sea, but mammals were relatively insignificant and presumed to have been predominantly nocturnal. The first birds, including Archaeopteryx, appeared in the Late Jurassic.

Jurassic One of the three Mesozoic periods, about 199.6–145.5 Ma ago, that followed the Triassic and preceded the Cretaceous. The Jurassic Period is subdivided into 11 stages, with clays, calcareous sandstones, and limestones being the most common rock types. Brachiopoda, Bivalvia, and Ammonoidea were abundant fossils, along with many other invertebrate stocks. Reptiles flourished on land and in the sea, but mammals were relatively insignificant and are presumed to have been predominantly nocturnal. The first birds, including Archaeopteryx, appeared in the Upper Jurassic.

Jurassic One of the three Mesozoic periods, about 208–145.6 Ma ago, which followed the Triassic and preceded the Cretaceous. The Jurassic Period is subdivided into 11 stages, with clays, calcareous sandstones, and limestones being the most common rock types. Brachiopoda, Bivalvia, and Ammonoidea were abundant fossils, along with many other invertebrate stocks. Reptiles flourished on land and in the sea, but mammals were relatively insignificant and are presumed to have been predominantly nocturnal. The first birds, including Archaeopteryx, appeared in the Late Jurassic.

Jurassic One of the three Mesozoic periods, about 213–144 Ma ago, that followed the Triassic and preceded the Cretaceous. The Jurassic Period is subdivided into 11 stages, with clays, calcareous sandstones, and limestones being the most common rock types. The first birds, including Archaeopteryx, appeared in the Upper Jurassic and the flora included many forms that are still extant (e.g. conifers, cycads, ferns, and ginkgos).

Ju·ras·sic / jəˈrasik/ • adj. Geol. of, relating to, or denoting the second period of the Mesozoic era, between the Triassic and Cretaceous periods. ∎  [as n.] (the Jurassic) the Jurassic period or the system of rocks deposited during it.

jurassic (geol.) pert. to oolitic formations of which the Jura mountains chiefly consist. XIX. — F. jurassique, f. Jura.

Jurassic •boracic, classic, Jurassic, neoclassic, potassic, thoracic, Triassic •ataraxic • carsick • heartsick •geodesic •anorexic, dyslexic •airsick • basic • seasick •extrinsic, intrinsic •fossick, virtuosic •toxic • homesick • lovesick