Jurassic

The Jurassic is a geologic period and stratigraphic system that lasted about 58.3 million years. It began 201.4 million years ago, at the end of the Triassic Period, and ended 143.1 million years ago, when the Cretaceous Period began. It is the second and middle period of the Mesozoic Era and the eighth period of the Phanerozoic Eon. It is named after the Jura Mountains, where limestone rocks from this period were first found.

The Jurassic began with the Triassic–Jurassic extinction event, a time when many living things died out. It was linked to the eruption of the Central Atlantic Magmatic Province (CAMP). Around 183 million years ago, the Toarcian Age started. It was marked by the Toarcian Oceanic Anoxic Event, a worldwide time when the oceans had very little oxygen, oceanic anoxia, the seas became more acidic, ocean acidification, and the world became very hot. This event was linked to more extinctions and was probably caused by the eruption of the Karoo-Ferrar large igneous provinces. The end of the Jurassic has no clear boundary with the Cretaceous. It is the only boundary between geological periods that is still not officially defined.

At the start of the Jurassic, the supercontinent Pangaea had begun to break apart, a process called rifting, into two landmasses: Laurasia in the north and Gondwana in the south. The climate was warmer than today's, and there were no ice caps. Forests grew close to the poles, while large dry areas spread across the lower latitudes.

On land, the animals changed. In the Triassic, animals were shared between dinosauromorph and pseudosuchian archosaurs. In the Jurassic, dinosaurs alone were the main animals. The first stem-group birds appeared during this period. They evolved from a branch of theropod dinosaurs. Other new developments included the earliest crabs and modern frogs, salamanders, and lizards. Mammaliaformes, one of the few cynodont lineages to survive the end of the Triassic, continued to grow in variety, and the first crown group mammals appeared. Crocodylomorphs moved from living on land to living in water. The oceans were home to marine reptiles such as ichthyosaurs and plesiosaurs. Pterosaurs were the most common flying vertebrates.

Modern sharks and rays first appeared and spread out during the Jurassic. The first known crown-group teleost fish, the main group of modern fish, appeared near the end of the period. Plant life was mostly ferns and gymnosperms, including conifers. Many modern plant groups first appeared at this time. Other groups appeared too, such as the extinct Bennettitales.

Etymology and history

The name "Jurassic" comes from the Jura Mountains. These are forested mountain ranges next to the Alps, mostly along the France–Switzerland border. In 1795, German naturalist Alexander von Humboldt noticed that the carbonate rocks in the Jura Mountains were different from the Triassic Muschelkalk rocks in southern Germany. He wrongly thought they were older. In 1799, he named them Jura-Kalkstein ('Jura limestone'). In 1829, French naturalist Alexandre Brongniart connected these rocks to similar rocks in Britain, called oolitic limestones, which were about the same age. He was the first to use the word "Jurassic."

In 1839, German geologist Leopold von Buch split the Jurassic into three parts: the Black Jurassic, Brown Jurassic, and White Jurassic, from oldest to youngest. Earlier, in 1822, William Conybeare and William Phillips had used the name "Lias" for rocks in England that were the same age as the Black Jurassic. Between 1842 and 1852, French palaeontologist Alcide d'Orbigny split the Jurassic into ten stages. He based these on ammonite fossils. Seven of these stages are still used today, but none of them keeps its original meaning. In 1858, German geologist and palaeontologist Friedrich August von Quenstedt divided von Buch's three parts in the Swabian Jura into six smaller parts. Between 1856 and 1858, German palaeontologist Albert Oppel changed d'Orbigny's system. He divided the stages into biostratigraphic zones, mostly based on ammonites. Most of the stages used today were officially set at a meeting in Luxembourg in 1962.

Geology

The Jurassic Period is split into three epochs: Early, Middle, and Late. In stratigraphy, the rock record is divided into the Lower Jurassic, Middle Jurassic, and Upper Jurassic series. Geologists sort Jurassic rocks into layers called stages. Each stage formed during a matching span of time, called an age. For worldwide comparisons, the International Commission on Stratigraphy (ICS) officially approves the global stages. Each stage has a marker called a Global Boundary Stratotype Section and Point (GSSP). It sits in a single formation (a stratotype) and shows where the stage begins. The ages of the Jurassic, from youngest to oldest, are listed below:

Scientists mostly use index fossils to date Jurassic rocks, especially ammonites. The first appearance datum of certain ammonite taxa marks the start of a stage. It also marks smaller spans of time within stages, called "ammonite zones." Worldwide, scientists use standard European ammonite zones. Other regions are matched to the European ones.

The oldest part of the Jurassic was once called the Lias or Liassic. It is about the same as the Early Jurassic, though it also includes part of the earlier Rhaetian. The Hettangian Stage was named in 1864 by Swiss scientist Eugène Renevier. The name comes from Hettange-Grande in north-eastern France. Its GSSP is at the Kuhjoch Pass in the Karwendel Mountains, in the Northern Calcareous Alps of Austria. It was officially approved in 2010. The Hettangian marks the start of the Jurassic. It begins with the first appearance of the ammonite Psiloceras spelae tirolicum in the Kendlbach Formation at Kuhjoch.

The Sinemurian Stage was introduced by Alcide d'Orbigny in 1842. It is named after the French town of Semur-en-Auxois, near Dijon. Its first definition included what is now the Hettangian. Its GSSP is on a cliff north of the hamlet of East Quantoxhead, 6 kilometres east of Watchet, in Somerset, England. It lies within the Blue Lias and was approved in 2000.

The Pliensbachian Stage was named in 1858 by Albert Oppel. It is named after the hamlet of Pliensbach in the community of Zell unter Aichelberg, in the Swabian Alb near Stuttgart, Germany. Its GSSP is at Wine Haven in Robin Hood's Bay, in Yorkshire, England. It lies in the Redcar Mudstone Formation and was approved in 2005.

The Toarcian Stage is named after the village of Thouars (Latin: Toarcium). The village is just south of Saumur in the Loire Valley of France. Alcide d'Orbigny named it in 1842. Its GSSP is at Peniche, Portugal. It was approved in 2014 and is marked by ammonites from the group Dactylioceras (Eodactylites).

The Aalenian is named after the German city of Aalen. Swiss geologist Karl Mayer-Eymar defined it in 1864. Its GSSP is at Fuentelsaz in the Iberian range, near Guadalajara, Spain. It was approved in 2000.

The Bajocian Stage was named by Alcide d'Orbigny in 1842. It is named after the town of Bayeux in Normandy, France. Its GSSP is at Cabo Mondego, Portugal, and was approved in 1997.

The Bathonian is named after the city of Bath, England. Belgian geologist d'Omalius d'Halloy introduced it in 1843. Its GSSP is in the Alpes de Haute Provence, France, and was approved in 2009.

The Callovian takes its name from the village of Kellaways in Wiltshire, England. Alcide d'Orbigny named it in 1852. Its base does not yet have an approved GSSP.

The Oxfordian is named after the city of Oxford in England. The name refers to the Oxford Clay. Alcide d'Orbigny named it in 1844. Its base also does not have a defined GSSP.

The Kimmeridgian takes its name from the village of Kimmeridge on the coast of Dorset, England. The name refers to the Kimmeridge Clay. Its GSSP is at Staffin Bay on the Isle of Skye, Scotland, and was approved in 2021.

The Tithonian's name comes from Greek mythology. This is unusual for geological stage names. Tithonus was the son of Laomedon of Troy. He fell in love with Eos, the Greek goddess of dawn. Albert Oppel chose this name because the Tithonian comes just before the Cretaceous begins, like the dawn. Its base also does not have a GSSP.

The upper boundary of the Jurassic is still not clearly defined. The Jurassic–Cretaceous boundary is the only system boundary without a defined GSSP. Most fossil markers vary a lot from place to place. There are also no chemostratigraphic events, such as sudden large changes in ratios of isotopes, that could mark it. Calpionellids have been suggested as the best candidates. They are a puzzling group of planktonic protists with urn-shaped calcite tests. They were briefly common in the latest Jurassic and earliest Cretaceous. The first appearance of Calpionella alpina has been proposed as the start of the Cretaceous. The working definition has often been the first appearance of the ammonite Strambergella jacobi. It was once placed in the genus Berriasella, though scientists have questioned how useful it is.

The Kimmeridge Clay and related rocks are the main source rock for North Sea oil. The Arabian Intrashelf Basin formed during the Middle and Late Jurassic. It holds the world's largest oil reserves, including the Ghawar Field, the largest oil field in the world. 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. Most are in the Turpan-Hami Basin and the Ordos Basin.

Some large impact events have left marks on Earth. The Morokweng impact structure is a 70 km wide impact area. It lies hidden beneath the Kalahari desert in northern South Africa. It is dated to the Tithonian, about 146.06 ± 0.16 Ma. Another large structure is the Puchezh-Katunki crater, which is 40 kilometres across. It is buried beneath Nizhny Novgorod Oblast in western Russia. It has been dated to the Sinemurian, 195.9 ± 1.0 Ma.

Paleogeography and tectonics

At the start of the Jurassic, almost all of the world's land was joined together in one huge continent called Pangaea. This supercontinent is a kind of supercontinent. During the Early Jurassic, it began to break apart into a northern part, Laurasia, and a southern part, Gondwana. The first split began between North America and Africa, and it was linked to the Central Atlantic Magmatic Province.

During the Jurassic, the northern Atlantic Ocean was fairly narrow. The southern Atlantic did not open until the Cretaceous. Oceans covered both the North and South Poles. The Tethys Ocean lay between Gondwana and Asia. Madagascar and Antarctica began to separate from Africa near the end of the Early Jurassic. By the Late Jurassic, North and South America had moved apart. Sea levels rose and fell over this time, and the oceans slowly changed their chemistry.

Climate

Main article: Toarcian Oceanic Anoxic Event

Main article: Tithonian extinction event

The Jurassic was generally warmer than today, by about 5–10 °C (9–18 °F). The atmospheric carbon dioxide level was likely about four times higher. Short "cold snap" periods still happened from time to time. Forests grew near the poles, with warm summers and cold, sometimes snowy winters. Clues such as dropstones and glendonites in northeastern Siberia point to cold winters. The deep ocean was likely 8 °C (14 °F) warmer than today, and coral reefs grew farther north and south. Large deserts and scrublands likely covered the areas near the equator, where the Intertropical Convergence Zone probably sat over the oceans. Tropical rainforest and tundra were likely rare or absent. How often wildfire happened followed a cycle of about 405,000 years linked to eccentricity. The breakup of Pangaea made the hydrological cycle much stronger.

The Jurassic began with rising temperatures. These were linked to the Triassic–Jurassic extinction and the eruption of the Central Atlantic magmatic province. An early cool period came next, followed by a sharp warm spike in the Toarcian. This spike was linked to the eruption of the Karoo-Ferrar large igneous provinces in southern Gondwana. During this warm spike, the oceans ran very low on oxygen. This event, called oceanic anoxia, can be seen in a change in carbon isotope and in dark layers of shales. Many sea animals died out, especially brachiopods. Others included ostracods, foraminifera, bivalves, and cnidarians. Possible release of methane clathrates and extra carbon dioxide likely added to the warming. The sea also became more acidic, a process called ocean acidification. In the Sichuan Basin, a giant lake formed. It was probably three times the size of today's Lake Superior, and it is now known from the Ziliujing Formation. Near the end of the period, there was a cool interval and a change in sea level. These affected wood and life during Early Cretaceous times.

Flora

Plants did not die out in a mass extinction at the end of the Triassic. In Greenland, fossil pollen and spores show that the types of plants changed completely. In Europe, the changes were mostly local, as ecological succession took place. The Peltaspermaceae disappeared from most of the world, but Lepidopteris lived on into the Early Jurassic in Patagonia. Dicroidium, a corystosperm seed fern, also became less common. Conifers became a major part of Jurassic forests. By the end of the Jurassic, most modern conifer groups had appeared. They grew from voltzialean ancestors. Araucarian conifers are first clearly recorded in the Early Jurassic. The extinct family Cheirolepidiaceae was also common, and Pararaucaria was one of its seed cones. The oldest clear fossil of the cypress family (Cupressaceae) comes from the Early Jurassic of Patagonia. By the Middle to Late Jurassic, cypresses were common in warm areas of the Northern Hemisphere, and modern kinds such as Sequoia first appeared. The pine family (Pinaceae) and yew family (Taxaceae) also have early forms. Podocarpaceae and the broad-leafed Podozamites were important too. The Ginkgoales have only one living species, Ginkgo biloba. In the Jurassic, they were much more varied. There were several groups, such as Yimaia and Karkenia, and the strap-shaped leaves of Umaltolepis.

The Bennettitales were common across both hemispheres. Their leaves look much like those of cycads, though the two groups are not closely related. Insects with long mouthparts, such as kalligrammatid lacewings and acrocerid flies, may have helped them spread pollen by feeding on nectar. Cycads reached their greatest variety during the Jurassic and Cretaceous. However, they were not the most common plants in the middle of the Mesozoic. There, Nilssoniales and Bennettitales were dominant. Some Jurassic cycads may be relatives of Cycadaceae and Zamiaceae. The widespread Ctenis group was distantly related to modern cycads. No clear Jurassic fossils of flowering plants are known. Today, they make up 90% of living plant species, and they became diverse in the Cretaceous. The earliest gnetophytes, one of the four main living groups of gymnosperms, appeared by the end of the Jurassic. Among the seed plant groups, the Caytoniales reached their peak. Their berry-like capsules have led some to suggest a link to flowering plants, but this is not proven. The Pentoxylales, a small and distinct group of liana-like plants, first appeared during the Jurassic.

Many fern families were widespread, including Dipteridaceae, Matoniaceae, Gleicheniaceae, Osmundaceae and Marattiaceae. The Cyatheales, which include most modern tree ferns, appeared in the Late Jurassic. Modern horsetails of the genus Equisetum first appear in the Early Jurassic. Equisetum thermale from Argentina has all the features of modern members of the genus. Quillworts that look almost identical to modern species are known from the Jurassic onward. Mosses and liverworts were also present. The moss Kulindobryum from Russia was found with dinosaur bones. It is thought to be related to the Splachnaceae, which grow on animal remains. The liverwort Pellites hamiensis from China is the oldest record of the Pelliaceae. Ricciopsis is the only Jurassic record of Ricciaceae.

Fauna

During the Jurassic, dinosaurs became the most common large animals on land after a mass extinction at the end of the Triassic. Sauropods were the main large plant-eaters on land, and some grew to huge sizes. Meat-eating theropods included early avialans, the group that contains birds, such as Archaeopteryx. Other reptiles also changed. Crocodylomorphs were the only group of their kind to survive the extinction, and they lived in many different ways. Some, such as the Thalattosuchia, lived in the sea. Ichthyosaurs were most varied in the Early Jurassic, and plesiosaurs and pterosaurs were also common. Among amphibians, lissamphibians began to spread, and early frogs and salamanders appeared. Small mammal relatives, the Mammaliaformes, spread widely and lived in many different ways.

Life in the water was also rich. Ray-finned fish were major parts of both freshwater and sea life, and sharks and rays were common. Many insects appeared, and beetles became very varied in the Middle to Late Jurassic. The extinction at the end of the Triassic removed many sea animals, and reefs later grew again in the Late Jurassic. Belemnites and octopuses lived in the seas, while crabs became common on reefs. Ammonites, which had been hit hard by the extinction, quickly spread out again in the Early Jurassic.