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 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 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 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.