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Jurassic Period, second of three periods of the Mesozoic Era. Extending from 201.3 million to 145 million years ago, it immediately followed the Triassic Period (251.9 million to 201.3 million years ago) and was succeeded by the Cretaceous Period (145 million to 66 million years ago). The Morrison Formation of the United States and the Solnhofen Limestone of German, both famous for their exceptionally well-preserved fossils, are geologic features that were formed during Jurassic times.
The distribution of landmasses, mountainous regions, shallow seas, and deep ocean basins during the late Jurassic Period. Included in the paleogeographic reconstruction are the locations of the interval's subduction zones.
The Jurassic was a time of significant global change in continental configurations, oceanographic patterns, and biological systems. During this period the supercontinent Pangea split apart, allowing for the eventual development of what are now the central Atlantic Ocean and the Gulf of Mexico. Heightened plate tectonic movement led to significant volcanic activity, mountain-building events, and attachment of islands onto continents. Shallow seaways covered many continents, and marine and marginal marine sediments were deposited, preserving a diverse set of fossils. Rock strata laid down during the Jurassic Period have yielded gold, coal, petroleum, and other natural resources.
During the Early Jurassic, animals and plants living both on land and in the seas recovered from one of the largest mass extinctions in Earth history. Many groups of vertebrate and invertebrate organisms important in the modern world made their first appearance during the Jurassic. Life was especially diverse in the oceans—thriving reef ecosystems, shallow-water invertebrate communities, and large swimming predators, including reptiles and squidlike animals. On land, dinosaurs and flying pterosaurs dominated the ecosystems, and birds made their first appearance. Early mammals also were present, though they were still fairly insignificant. Insect populations were diverse, and plants were dominated by the gymnosperms, or “naked-seed” plants.
The Jurassic Period was named early in the 19th century, by the French geologist and mineralogist Alexander Brongniart, for the Jura Mountains between France and Switzerland. Much of the initial work by geologists in trying to correlate rocks and develop a relative geologic time scale was conducted on Jurassic strata in western Europe.
Although the breakup of the supercontinent Pangea had already started in the Triassic Period, the continents were still very close together at the beginning of Jurassic time. The landmasses were grouped into a northern region—Laurasia—consisting of North America and Eurasia, and a southern region— Gondwana —consisting of South America, Africa, India, Antarctica, and Australia. These two regions were separated by Tethys, a tropical east-west seaway. During the Jurassic, spreading centres and oceanic rifts formed between North America and Eurasia, between North America and Gondwana, and between the various segments of Gondwana itself (see the map). In the steadily opening, though still restricted, ocean basins, there was a continuous accumulation of thick flood basalts and a subsequent deposition of sediments. Some of these deposits, such as salt deposits in the Gulf of Mexico and oil-bearing shales of the North Sea, are economically important today. In addition to ocean basin spreading, continental rifting initiated during the Jurassic, eventually separating Africa and South America from Antarctica, India, and Madagascar. The numerous microplates and blocks making up the complex Caribbean region today can be traced to this time interval.
Pangea : Late Jurrasic Period
Paleogeography and paleoceanography of Late Jurassic time. Present-day coastlines and tectonic boundaries of continents are shown in the inset at the lower right.
To accommodate the production of new seafloor along the proto-Atlantic Ocean, significant subduction zones (where seafloor is destroyed) were active along virtually all the continental margins around Pangea as well as in southern Tibet, southeastern Europe, and other areas. All along the west coast of North, Central, and South America, plate tectonic activity in the subduction zones brought on the initial formation of north-south mountain ranges such as the Rocky Mountains and the Andes. Along western North America, several terranes (islands or microcontinents riding on a moving plate) were brought east on oceanic crust and collided with the continent, including parts of a microcontinent that collided into the Alaskan and Siberian regions in the northern Pacific. These collisions added to the growth of the North American continent and its mountain chains. One mountain-building event, known as the Nevadan orogeny, resulted in the emplacement of massive igneous and metamorphic rocks from Alaska to Baja California. Granites formed in the Sierra Nevadas during this time can be seen today in Yosemite National Park, California.
In the Early Jurassic the western interior of North America was covered by a vast sand sea, or erg—one of the largest deposits of dune sands in the geologic record. These deposits (including the Navajo Sandstone) are prominent in a number of places today, including Zion National Park, Utah. In Middle and early Late Jurassic times, the western regions of North America were covered by shallow seaways that advanced and retreated repeatedly, leaving successive accumulations of marine sandstones, limestones, and shales. By Late Jurassic time the seaway had retreated, and strata bearing dinosaur fossils were deposited in river floodplains and sream channel environments, such as those recorded in the Morrison Formation, Montana.
Records of sea level changes can be found on every continent. However, because of the significant tectonic activity occurring around the world, it is not clear which of these local changes can be correlated to global sea level change. Because there is no evidence of major glaciations in the Jurassic, any global sea level change must have been due to thermal expansion of seawater or plate tectonic activity (such as major activity at seafloor ridges). Some geologists have proposed that average sea levels increased from Early to Late Jurassic time.
Jurassic climates can be reconstructed from the analyses of fossil and sediment distribution and from geochemical analyses. Fossils of warm-adapted plants are found up to 60° N and 60° S paleolatitude, suggesting an expanded tropical zone. In higher paleolatitudes, ferns and other frost-sensitive plants indicate that there was a less severe temperature difference between the Equator and the poles than exists today. Despite this decreased temperature gradient, there was a marked difference in marine invertebrates from northern higher latitudes—the boreal realm—and the tropical Tethyan realm. Decreased latitudinal temperature gradients probably led to decreased zonal winds.
Large salt deposits dating from the Jurassic represent areas of high aridity, while extensive coal deposits suggest areas of high precipitation. It has been suggested that an arid belt existed on the western side of Pangea, while more-humid conditions existed in the east. These conditions may have been caused by large landmasses affecting wind and precipitation in a manner similar to that of modern continents.
Analyses of oxygen isotopes in marine fossils suggest that Jurassic global temperatures were generally quite warm. Geochemical evidence suggests that surface waters in the low latitudes were about 20 °C (68 °F), while deep waters were about 17 °C (63 °F). Coolest temperatures existed during the Middle Jurassic and warmest temperatures in the Late Jurassic. A drop in temperatures occurred at the Jurassic-Cretaceous boundary.
It has been suggested that increased volcanic and seafloor-spreading activity during the Jurassic released large amounts of carbon dioxide—a greenhouse gas—and led to higher global temperatures. Warm temperatures and decreased latitudinal gradients also may be related to the Tethys Sea, which distributed warm, tropical waters around the world. Ocean circulation was probably fairly sluggish because of the warm temperatures, lack of ocean density gradients, and decreased winds. As stated above, there is no evidence of glaciation or polar ice caps in the Jurassic. This may have been caused by the lack of a continental landmass in a polar position or by generally warm conditions; however, because of the complex relationships between temperature, geographic configurations, and glaciations, it is difficult to state a definite cause and effect.
The Triassic-Jurassic boundary is marked by one of the five largest mass extinctions on earth. About half of the marine invertabrate genera went extinct at this time; whether land plants or terrestrial vertebrates suffered a similar extinction during this interval is unclear. In addition, at least two other Jurassic intervals show heightened faunal turnover affecting mainly marine invertebrates—one in Early Jurassic time and another at the end of the period.
Jurassic rock strata preserve the first appearances of many important modern biological groups. In the oceans, life on the seafloor became more complex and modern, with an abundance of mollusks and coral reef builders by Middle Jurassic time. While modern fishes became common in Jurassic seas, they shared the waters with ammonites and other squidlike organisms as well as large reptiles that are all extinct today. On land a new set of plants and animals was dominant by the Early Jurassic. Gymnosperms (“naked-seed” plants such as conifers) replaced the seed ferns that dominated older ecosystems. Similarly, dinosaurs and mammals, as well as amphibians and reptiles resembling those of modern times, replaced the ancestral reptiles and mammal groups common in Late Triassic times.
The earliest bird fossils were found in Jurassic rocks. However, although groups now living were present in Jurassic terrestrial ecosystems, Jurassic communities would still have been very different because dinosaurs were the dominant animals.
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