Jurassic

The Jurassic is a geologic period and system that spanned 56 million years from the end of the Triassic Period  million years ago (Mya) to the beginning of the Cretaceous Period  Mya. The Jurassic constitutes the middle period of the Mesozoic Era. The Jurassic is named after the Jura Mountains in the European Alps, where limestone strata from the period were first identified.

The start of the period was marked by the major Triassic–Jurassic extinction event. Two other extinction events occurred during the period: the Pliensbachian-Toarcian extinction in the Early Jurassic, and the Tithonian event at the end; neither event ranks among the "Big Five" mass extinctions, however.

The Jurassic period is divided into three epochs: Early, Middle, and Late. Similarly, in stratigraphy, the Jurassic is divided into the Lower Jurassic, Middle Jurassic, and Upper Jurassic series of rock formations.

By the beginning of the Jurassic, the supercontinent Pangaea had begun rifting into two landmasses: Laurasia to the north, and Gondwana to the south. This created more coastlines and shifted the continental climate from dry to humid, and many of the arid deserts of the Triassic were replaced by lush rainforests.

On land, the fauna transitioned from the Triassic fauna, dominated by both dinosauromorph and crocodylomorph archosaurs, to one dominated by dinosaurs alone. The first birds also appeared during the Jurassic, having evolved from a branch of theropod dinosaurs. Other major events include the appearance of the earliest lizards, and the evolution of therian mammals. Crocodilians made the transition from a terrestrial to an aquatic mode of life. The oceans were inhabited by marine reptiles such as ichthyosaurs and plesiosaurs, while pterosaurs were the dominant flying vertebrates.

Etymology
The chronostratigraphic term "Jurassic" is directly linked to the Jura Mountains, a mountain range mainly following the course of the France–Switzerland border. During a tour of the region in 1795, Alexander von Humboldt recognized the mainly limestone dominated mountain range of the Jura Mountains as a separate formation that had not been included in the established stratigraphic system defined by Abraham Gottlob Werner, and he named it "Jura-Kalkstein" ('Jura limestone') in 1799.

Thirty years later, in 1829, the French naturalist Alexandre Brongniart published a survey on the different terrains that constitute the crust of the Earth. In this book, Brongniart referred to the terrains of the Jura Mountains as terrains jurassiques, thus coining and publishing the term for the first time.

The name "Jura" is derived from the Celtic root *jor via Gaulish *iuris "wooded mountain", which, borrowed into Latin as a place name, evolved into Juria and finally Jura.

Geology
The Jurassic period is divided into three epochs: Early, Middle, and Late. Similarly, in stratigraphy, the Jurassic is divided into the Lower Jurassic, Middle Jurassic, and Upper Jurassic series of rock formations, also known in Europe as Lias, Dogger and Malm. The division of the Jurassic into three parts originated with Leopold von Buch. The three epochs are subdivided into shorter spans of time called ages. The ages of the Jurassic from youngest to oldest are:

Stratigraphy
Jurassic stratigraphy is primarily based around of the use of ammonites as index fossils, with the First Appearance Datum of specific ammonite taxa being used to mark the beginnings of stages, and well as smaller timespans within stages, referred to as "Ammonite Zones", these in turn are also sometimes subdivided further into subzones. Global stratigraphy is based on standard European ammonite zones, with other regions being calibrated to the European successions. The oldest part of the Jurassic period has historically been referred to as the Lias or Liassic, roughly equivalent in extent to the Early Jurassic, but also including part of the preceding Rhaetian. The Hettangian stage was named by Swiss palaeontologist Eugène Renevier in 1864 after Hettange-Grande in North-Eastern France. The Global Boundary Stratotype Section and Point (GSSP) for the base of the Hettangian is located at Kuhjoch pass, Karwendel Mountains, Northern Calcareous Alps, Austria, which was ratified in 2010. The beginning of the Hettangian, and thus the Jurassic as a whole, is marked by the first appearance of the ammonite Psiloceras spelae tirolicum. The base of the Jurassic was previously defined as the first appearance of Psiloceras planorbis by Albert Oppel in 1856-58, but this was changed as the appearance was seen as too localised an event for an international boundary.

The Sinemurian stage was defined and introduced into scientific literature by French palaeontologist Alcide d'Orbigny in 1842. It takes its name from the French town of Semur-en-Auxois, near Dijon. The original definition of Sinemurian included what is now the Hettangian. The GSSP of the Sinemurian is located at a cliff face north of the hamlet of East Quantoxhead, 6 kilometres east of Watchet, Somerset, England, within the Blue Lias. The beginning of the Sinemurian is defined by the first appearance of the ammonite Vermiceras quantoxense.

The Pleinsbachian was named by German palaeontologist Albert Oppel in 1858 after the hamlet of Pliensbach in the community of Zell unter Aichelberg in the Swabian Alb, near Stuttgart, Germany. The GSSP for the base of the Pleinsbachian is found at the Wine Haven locality in Robin Hood's Bay, Yorkshire, England, in the Redcar Mudstone Formation. The beginning of the Pleinsbachian is defined by the first appearance of the ammonite Bifericeras donovani.

The Toarcian is named after the village Thouars (Latin: Toarcium), just south of Saumur in the Loire Valley of France, it was defined by Alcide d'Orbigny in 1842 originally from Vrines quarry around 2 km northwest of the village. The GSSP for the base of the Toarcian is located at Peniche, Portugal. The boundary is defined by the first appearance of ammonites belonging to the subgenus Dactylioceras (Eodactylioceras).

The Aalenian is named after the city of Aalen in Germany. The Aalenian was defined by Swiss geologist Karl Mayer-Eymar in 1864. The lower boundary was originally between the dark clays of the Black Jurassic and the overlying clayey sandstone and ferruginous oolite of the Brown Jurassic sequences of southwestern Germany. The GSSP for the base of the Aalenian is located at Fuentelsaz in the Iberian range near Guadalajara, Spain. The base of the Aalenian is defined by the first appearance of the ammonite Leioceras opalinum.

The Bajocian is named after the town of Bayeux (Latin: Bajoce) in Normandy, France, and was defined by Alcide d'Orbigny in 1842. The GSSP for the base of the Bajocian is located at Murtinheira in Portugal, and was defined in 1997. The base of the Bajocian is defined by the first appearance of the ammonite Hyperlioceras mundum.

The Bathonian is named after the city of Bath, England, introduced by Belgian geologist d'Omalius d'Halloy in 1843, after an incomplete section of oolitic limestones in several quarries in the region. The GSSP for the base of the Bathonian is Ravin du Bès, Bas-Auran area, Alpes de Haute Provence, France, which was defined in 2009. The base of the Bathonian is defined by the first appearance of the ammonite Gonolkites convergens, at the base of the Zigzagiceras zigzag ammonite zone.

The Callovian is derived from the latinized name of the village of Kellaways in Wiltshire, England, and was defined by Alcide d'Orbigny in 1852, originally with base at the contact between the Forest Marble Formation and the Cornbrash Formation. However, this boundary was later found to be situated within the upper part of the Bathonian. The base of the Callovian does not yet have a certified GSSP, as of 2019.

The Oxfordian is named after the city of Oxford in England, and was named by Alcide d'Orbigny in 1844 in reference to the Oxford Clay. The base of the Oxfordian lacks a defined GSSP. W. J. Arkell in studies in 1939 and 1946 placed the lower boundary of the Oxfordian as the first appearance of the ammonite Quenstedtoceras mariae (then placed in the genus Vertumniceras). Subsequent proposals have suggested the first appearance of Cardioceras redcliffense as the lower boundary.

The Kimmeridgian is named after the village of Kimmeridge on the coast of Dorset, England. It was named by Alcide d'Orbigny in 1842, in reference to the Kimmeridge Clay. Although not confirmed, the Flodigarry section at Staffin Bay on the Isle of Skye, Scotland has been submitted as the GSSP for the base of the Kimmeridgian.

The Tithonian was introduced in scientific literature by Albert Oppel in 1865. The name Tithonian is unusual in geological stage names because it is derived from Greek mythology rather than a placename. Tithonus was the son of Laomedon of Troy and fell in love with Eos, the Greek goddess of dawn. His name was chosen by Albert Oppel for this stratigraphical stage because the Tithonian finds itself hand in hand with the dawn of the Cretaceous. The base of the Tithonian currently lacks a GSSP. The upper boundary of the Jurassic is also currently undefined. Calpionellids, an enigmatic group of pelagic protists with urn shaped calcitic tests briefly abundant during the latest Jurassic to earliest Cretaceous, have been suggested to represent the most promising candidates for fixing the J/K boundary.

Mineral and Hydrocarbon deposits
The Kimmeridge Clay and equivalents are the major source rock for the North Sea oil. The Arabian Intrashelf Basin, deposited from the late Middle to Upper Jurassic, is the setting of the worlds largest oil reserves, including the Ghawar Field, the world largest oil field. The Jurassic aged Sargelu and Naokelekan Formations are major source rocks for oil in Iraq. Over 1500 gigatons of Jurassic coal reserves are found in North-West China, primarily in the Turpan-Hami Basin and the Ordos Basin.

Impact craters
Major impact craters include the Morokweng crater, a 70 km diameter crater buried beneath the Kalahari desert in northern South Africa. The impact is dated to the Jurassic-Cretaceous boundary, around 145 Ma. The Morokweng crater has been suggested to have had a role in the turnover at the Jurassic-Cretaceous transition. Another major impact crater is the Puchezh-Katunki crater, 40-80 kilometres in diameter, buried beneath Nizhny Novgorod Oblast, Russia. The impact has been dated to the Sinemurian, around 192-196 Mya.

Paleogeography and tectonics
During the early Jurassic period, the supercontinent Pangaea broke up into the northern supercontinent Laurasia and the southern supercontinent Gondwana; the Gulf of Mexico opened in the new rift between North America and what is now Mexico's Yucatán Peninsula. The Jurassic North Atlantic Ocean was relatively narrow, while the South Atlantic did not open until the following Cretaceous period, when Africa and South America rifted apart. The continents were surrounded by Panthalassa, with the Tethys Ocean between Gondwana and Asia. Western and Central Europe formed an archipelago of islands surrounded by shallow seas. Madagascar and Antarctica began to rift away from Africa during Early Jurassic, beginning the fragmentation of Gondwana. Climates were warm, with no evidence of a glacier having appeared. As in the Triassic, there was apparently no land over either pole, and no extensive ice caps existed.

Based on estimated sea level curves, the sea level was close to present levels during the Hettangian and Sinemurian, rising several tens of metres during the late Sinemurian-Pleinsbachian, before regressing to near present levels by the late Pleinsbachian. There seems to have been a gradual rise to a peak of ~75 m above present sea level during the Toarcian. During the latest part of the Toacian, the sea level again drops by several tens of metres. The sea level progressively rose from the Aalenian onwards, aside from dips of a few tens of metres in the Bajocian and around the Callovian-Oxfordian boundary, culminating in a sea level possibly as high as 140 metres above present sea level at the Kimmeridgian-Tithonian boundary. The sea levels falls in the Late Tithonian, perhaps to around 100 metres, before rebounding to around 110 metres at the Tithonian-Berriasian boundary.

The Jurassic geological record is good in western Europe, where extensive marine sequences indicate a time when much of that future landmass was submerged under shallow tropical seas; famous locales include the Jurassic Coast World Heritage Site in southern England and the renowned late Jurassic lagerstätten of Holzmaden and Solnhofen in Germany. In contrast, the North American Jurassic record is the poorest of the Mesozoic, with few outcrops at the surface. Though the epicontinental Sundance Sea left marine deposits in parts of the northern plains of the United States and Canada during the late Jurassic, most exposed sediments from this period are continental, such as the alluvial deposits of the Morrison Formation.

The Jurassic was a time of calcite sea geochemistry in which low-magnesium calcite was the primary inorganic marine precipitate of calcium carbonate. Carbonate hardgrounds were thus very common, along with calcitic ooids, calcitic cements, and invertebrate faunas with dominantly calcitic skeletons.

The first of several massive batholiths were emplaced in the northern American cordillera beginning in the mid-Jurassic, marking the Nevadan orogeny.

In Africa, Early Jurassic strata are distributed in a similar fashion to Late Triassic beds, with more common outcrops in the south and less common fossil beds which are predominated by tracks to the north. As the Jurassic proceeded, larger and more iconic groups of dinosaurs like sauropods and ornithopods proliferated in Africa. Middle Jurassic strata are neither well represented nor well studied in Africa. Late Jurassic strata are also poorly represented apart from the spectacular Tendaguru fauna in Tanzania. The Late Jurassic life of Tendaguru is very similar to that found in western North America's Morrison Formation.

Flora
The arid, continental conditions characteristic of the Triassic steadily eased during the Jurassic period, especially at higher latitudes; the warm, humid climate allowed lush jungles to cover much of the landscape. Gymnosperms were relatively diverse during the Jurassic period. Conifers in particular dominated the flora, as during the Triassic; they were the most diverse group and constituted the majority of large trees.

Extant conifer families that flourished during the Jurassic included the Araucariaceae, Cephalotaxaceae, Pinaceae, Podocarpaceae, Taxaceae and Taxodiaceae. The extinct Mesozoic conifer family Cheirolepidiaceae dominated low latitude vegetation, as did the shrubby Bennettitales. Cycads, similar to palm trees, 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 sized. Ginkgo plants were particularly common in the mid- to high northern latitudes. In the Southern Hemisphere, podocarps were especially successful, while Ginkgos and Czekanowskiales were rare.

In the oceans, modern coralline algae appeared for the first time.

Aquatic and marine
During the Jurassic period, the primary vertebrates living in the sea were fish and marine reptiles. The latter include ichthyosaurs, which were at the peak of their diversity, plesiosaurs, including pliosaurs, and marine thalattosuchian crocodyliformes of the families Teleosauridae, Machimosauridae and Metriorhynchidae. During the Late Jurassic, filter feeding pachycormiform fish would begin to diversify, including the largest bony fish known to have existed, Leedsichthys, with an estimated maximum length of over 15 metres. During the Middle Jurassic, the oldest known mackerel sharks appeared, represented by the genus Palaeocarcharias.

In the invertebrate world, several new groups appeared, including rudists (a reef-forming variety of bivalves) and belemnites. Calcareous sabellids (Glomerula) appeared in the Early Jurassic. The Jurassic also had diverse encrusting and boring (sclerobiont) communities, and it saw a significant rise in the bioerosion of carbonate shells and hardgrounds. Especially common is the ichnogenus (trace fossil) Gastrochaenolites. Ammonites were abundant throughout the Jurassic, all Jurassic ammonites originate from the Phylloceratina, the only ammonite group to have surivived the T-J extinction.

During the Jurassic period, about four or five of the twelve clades of planktonic organisms that exist in the fossil record either experienced a massive evolutionary radiation or appeared for the first time.

Terrestrial
On land, various archosaurian reptiles remained dominant. The Jurassic was a golden age for the large herbivorous dinosaurs known as the sauropods—Camarasaurus, Apatosaurus, Diplodocus, Brachiosaurus, and many others—that roamed the land late in the period; their foraging grounds were either the prairies of ferns, palm-like cycads and bennettitales, or the higher coniferous growth, according to their adaptations. The smaller Ornithischian herbivore dinosaurs, like stegosaurs and small ornithopods were less predominant, but played important roles. They were preyed upon by large theropods, such as Ceratosaurus, Megalosaurus, Torvosaurus and Allosaurus, all these belong to the 'lizard hipped' or saurischian branch of the dinosaurs.

During the Late Jurassic, the first avialans, like Archaeopteryx, evolved from small coelurosaurian dinosaurs. In the air, pterosaurs were common; they ruled the skies, filling many ecological roles now taken by birds, and may have already produced some of the largest flying animals of all time. Within the undergrowth were various types of early mammals, as well as tritylodonts, lizard-like sphenodonts, and early lissamphibians. The rest of the Lissamphibia evolved in this period, introducing the first salamanders and caecilians.

Turtles
Stem-group turtles (Testudinata) would diversify during the Jurassic. Jurassic stem-turtles belong to two progessively more advanced clades, the Mesochelydia and Perichelydia. It is thought that the ancestral condition for Mesochelydians is aquatic, as opposed to terrestial for Testudinata. The two modern groups of turtles (Testudines), Pleurodires and Cryptodires, would diverge by the Middle Jurassic. Early cryptodire lineages like Xinjiangchelyidae are known from the Middle Jurassic, while an early stem pleurodire lineage, the Platychelyidae is known from the Late Jurassic.

Lepidosaurs
Lepidosaurs, which include squamates (lizards and snakes) and rhynchocephalians (which today only includes the Tuatara) would diversify during the Jurassic. Rynchocephalians would occupy a wide range of morphologies and lifestyles during the Jurassic, such as the specialised aquatic Pleurosauridae and well as the herbivorous Opisthodontia. Squamates would also diversify during the Jurassic, with the estimated origin of living lizards during the Early Jurassic (~190 Mya) and the divergence of most major squamatan groups during the Early-Middle Jurassic. Many Jurassic squamates have unclear relationships to living groups. Early members of the snake lineage Ophidia appear during the Middle Jurassic.

Amphibians
The vast majority of temnospondyls went extinct at the end of the Triassic, with only brachyopoids surviving into the Jurassic and beyond. Members of the family Brachyopidae are known from Jurassic deposits in Asia, while the chigutisaurid Siderops is known from the Early Jurassic of Australia. Modern lissamphibians would also begin to diversify during the Jurassic. The Early Jurassic Prosalirus thought to represent the first frog relative with a morphology capable of hopping like living frogs. Morphogically recognisable stem-frogs like Notobatrachus are known from the Middle Jurassic. While the earliest salamander-line amphibians are known from the Triassic. Crown group salamanders first appear during the Middle-Late Jurassic of Eurasia, alongside stem-group relatives. Many Jurassic stem-group salamanders, like Marmorerpeton and Kokartus are thought to have been neotenic. Early representatives of crown group salamanders include Chunerpeton, Pangerpeton and Linglongtriton from the Middle-Late Jurassic Yanliao Biota of China, which belong to the Cryptobranchoidea, which contains living asiatic and giant salamanders. While Beiyanerpeton, also from the same biota is thought to be an early member of Salamandroidea, the group which contains all other living salamanders. Salamanders would disperse into North America by the end of the Jurassic, as evidenced by Iridotriton found in the Late Jurassic Morrison Formation. The oldest undisputed stem-caecilian is the Early Jurassic Eocaecilia from Arizona. The fourth group of lissamphibians, the extinct albanerpetontids, would first appear in the Middle Jurassic, represented by Anoualerpeton priscus from the Bathonian of Britain, as well as indeterminate remains from the equivalently aged Anoual Formation of Morocco.

Mammals
Mammals diversified extensively during the Jurassic, important groups of Jurassic mammals include Morganucodonta, Docodonta, Eutriconodonta, Dryolestida, Haramiyida and Multituberculata. While most Jurassic mammals are solely known from isolated teeth and jaw fragments, exceptionally preserved remains have revealed a variety of lifestyles. The docodontan Castorocauda was adapted for aquatic life, similar to the platypus and otters. Some members of Haramiyida and the eutriconodontan tribe Volaticotherini possessed a patagium akin to those of flying squirrels, allowing them to glide through the air. Fruitafossor was likely a specialist on colonial insects, similar to living anteaters. Non-mammalian cynodonts of the family Tritylodontidae survived the T-J extinction, and continued to exist through the Early Cretaceous. Theriiform mammals, represented today by living placentals and marsupials, would also appear during the early Late Jurassic, represented by Juramaia, a eutherian mammal closer to the ancestry of placentals than marsupials. Juramaia is much more advanced than expected for its age, as other theriiform mammals do not appear until the Early Cretaceous.

Insects and arachnids
Numerous important insect fossil localities are known from the Jurassic of Eurasia, the most important being the Karabastau Formation of Kazakhstan, and the various Yanliao Biota deposits in Inner Mongolia, China, such as the Daohugou Bed, dating to the Callovian-Oxfordian. The diversity of insects was stagnant throughout the Early and Middle Jurassic, but during the latter third of the Jurassic origination rates increased substantially while extinction rates remained flat. The Middle-Late Jurassic was a time of major diverisification for beetles. Weevils first appear in the fossil record during the Middle-Upper Jurassic, but are suspected to have originated during the Late Triassic-Early Jurassic. The oldest known lepidopterans (the group containing butterflies and moths) are known from the Triassic-Jurassic boundary, with wing scales belonging to the suborder Glossata and Micropterigidae-grade moths from the deposits of this age in Germany. Although modern representatives are not known until the Cenozoic, ectoparasitic insects thought to represent a stem-group to fleas first appear during the Jurassic, such as Pseudopulex jurassicus. These insects are substantially different from modern fleas, lacking the specialised morphology of modern fleas and being larger in size. The earliest group of Phasmatodea (stick insects), the winged Susumanioidea, an outgroup to living Phasmatodeans, first appear during the Middle Jurassic. The oldest member of the Mantophasmatidae (gladiators) also appeared during this time.

Only a handful of records of mites are known from the Jurassic, including Jureremus, an Oribatid mite belonging to the family Cymbaeremaeidae known from the Upper Jurassic of Britain and Russia. Spiders would diversify through the Jurassic. The Early Jurassic Seppo koponeni thought to possibly represent a stem-group to Palpimanoidea. Eoplectreurys from the Middle Jurassic of China is considered a stem lineage of Synspermiata. The oldest member of the family Archaeidae, Patarchaea, is known from the Middle Jurassic. Mongolarachne from the Middle Jurassic of China is among the largest known fossil spiders, with legs over 5 centimetres in length. The only scorpion known from the Jurassic is Liassoscorpionides from the Lower Jurassic of Germany, of uncertain placement. Eupnoi Opiliones are known from the Middle Jurassic, including members of the family Sclerosomatidae.

In popular culture
Since the early 1990s, the term Jurassic has been popularised by the Jurassic Park franchise, which started in 1990 with Michael Crichton's novel of the same title and its film adaptation, first released in 1993.