Ordovician

The Ordovician is the second chronostratigraphic system ( or period in geochronology ) of the Paleozoic in the Earth's history. The Ordovician began about 485.4 million years ago and ended about 443.4 million years. It is underlain by the Cambrian, the layers of the Cambrian are therefore the oldest of the Paleozoic. In the Ordovician, the Silurian follows.

History and naming

The term Ordovician was introduced in 1879 by the British geologist Charles Lapworth. He derived it from the Ordovicern, a Celtic tribe which was resident in Wales. In the 19th century, today Ordovician was mostly attributed to lower unit as the Silurian. To a dispute between supporters of Adam Sedgwick, the discoverer of the Cambrian, and Roderick Murchison, the descriptor of the Silurian to arbitrate, introduced Charles Lapworth in 1879 for the layers that have been claimed by both for their respective systems, the term Ordovician. He had recognized that these layers, quite clearly distinguished by their fossils, from the other two systems. But the new system name between Cambrian and Silurian sat through only slowly and only in 1960 was recognized by the International Geological Congress worldwide.

Definition and GSSP

The base of the Ordovician is defined by the International Union of Geological Sciences ( IUGS ​​) by the first appearance of the conodont species Iapetognathus fluctivagus. This limit is slightly above the Cordylodus lindstromi - conodont zone and slightly below the Erstauftretens the first planktonic graptolites ( Staurograptus dichotomus and Rhabdinopora praeparabola ). The upper limit ( = lower limit of the Siluriums ) was defined as the first appearance of the graptolite - type Akidograptus ascensus; the first appearance of the graptolite - type Parakidograptus acuminatus is only slightly higher and therefore already in the lowest stage of the Silurian. The GSSP ( global calibration point ) for the beginning of the Ordovician (and the Tremadocian stage) is the " Green Point Profile" in Gros Morne National Park, about 70 km from the airport of Deer Lake and about 10 kilometers north of the town Rocky Harbour, western Newfoundland ( Canada).

Subdivision of the Ordovician

The Ordovician is subdivided lower, middle and Oberordovizium into three chronostratigraphic series. These series are further divided into a total of seven stratigraphic levels (summary ):

• System: Ordovician ( 485.4 to 443.4 mya ) Series: Oberordovizium ( Upper Ordovician ) ( 458.4 to 443.4 mya ) Level: Hirnantium ( 445.2 to 443.4 mya )
• Level: Katium ( 453 to 445.2 mya )
• Level: Sandbium ( 458.4 to 453 mya )

• Level: Darriwilium ( 467.3 to 458.4 mya )
• Level: Dapingium ( 470 to 467.3 mya )

• Level: Floium ( 477.7 to 470 mya )
• Level: Tremadocian ( 485.4 to 477.7 mya )

Were a departure in England following regional series and stages for the Ordovician used:

• System: Ordovician Regional series: Ashgill Regional (and global ) level Hirnantian
• Regional level: Rawtheyan
• Regional level: Cautleyan
• Regional level: Pusgillian

• Regional level: Streffordian
• Regional level: Cheneyum
• Regional level: Burrellian
• Regional level: Aurelucian

• Regional level: Llandeilian

• Regional level: Fennian
• Regional level: Whitlandian
• Regional level: Moridunian

• Regional level: Migneintian
• Regional level: Cressagian

In the older German literature the British series were used as stages:

• Ordovician system Ashgilium
• Caradocium
• Llandeilium
• Arenigium
• Tremadocian

Especially in the older German literature of the 19th century, the Ordovician is also called Untersilur ( see history) referred. This can lead to confusion and false temporal classifications.

Paleogeography

The paleogeographic situation of the continents was marked as in the Cambrian from the Gondwana supercontinent and three other smaller continents Laurentia, Baltica and Sibiria as well as a number of small and micro- continents, which are part of Gondwana were originally. Baltica and Gondwana during the Ordovician something away from each other, and in between was the Tornquist Ocean. Laurentia drifted north to the Equator. It was separated from Gondwana and Baltica by the Iapetus Ocean. Sibiria was already in Mittelordovizium at the equator.

Based on the present continents wandered the south pole of a position in today's southern Algeria ( Unterordovizium ) initially somewhat to the north to about the present-day Mediterranean coast of Algeria ( Mittelordovizium ), and then hike up to Oberordovizium to West Africa on; more correctly migrated Gondwana accordingly across the South Pole. The North Pole was in the then world-wide Panthalassischen Ocean.

In Unterordovizium the microcontinent Avalonia from the northern edge of Gondwana broke off and drifted to the north. Between Avalonia and Gondwana, the Rheische Ocean opened. Avalonia formed in Mittelordovizium own faunal province that differed from that of Gondwana, Baltica and Laurentia. In Oberordovizium Avalonia was welded under closure of the Tornquist Ocean - to Baltica and Avalonia and Baltica between the Faunenunterschiede disappeared. The Iapetus Ocean began to close between Laurentia and Baltica. Probably also still in Unterordovizium the micro Perunica continent of Gondwanaland was canceled and also drifted north on Baltica.

In Oberordovizium ( Hirnantium ) froze a large portion of Gondwana ( North Africa, South America, Saudi Arabia). On the continents affected tillites were deposited. By glacier scrapes bedrock allows the transport direction of the ice reconstruct. In the adjacent marine areas, there were deposits of sediments with dropstones. Dropstones occur when falling into icebergs frozen coarse sediment by the melting of icebergs in mostly fine-grained sediments. The sediment was taken up by inland glaciers from the substrate and transported to the coast. There broke repeatedly from large parts and exaggerated as icebergs on the adjacent seas. The micro- continents of Armorica group ( as part of the Hun - Superterrans ) that are important for Europe later, still lay on the northern edge of Gondwana.

Climate

At the beginning of the Ordovician it was probably very warm near the equator. Also from the regions of the then South Pole are no known icing. However, at the end of the Ordovician ( Hirnantium stage), there was one of the largest glaciations throughout the Äonothems the Phanerozoic. This icing affected a large part of the southern hemisphere.

According to a study published in 2012 that took place in this period appearance of land plants was responsible for a strong reduction of atmospheric carbon. Since the land plants withdrawn to grow the bottom calcium, magnesium, phosphorus and iron, they create a chemical decomposition of the soil, which in turn was removed from the atmosphere of carbon dioxide. The associated global cooling by about 5 degrees led to a mass extinction.

Development of the fauna

At the end of the Cambrian period there had been a widespread regression and many species had become extinct. Including some of the early experiments of arthropods ( Arthropoda ) as the Anomalocarida were ( Anomalocaris ).

In Unterordovizium but it came to a renewed Radiation. The corals occur for the first time with the two groups of Rugosa and tabulata in appearance. The graptolites have their first appearance at the beginning of the Ordovician. As the last of the great tribes of the animal kingdom and the bryozoans ( Bryozoa ) and has been in a considerable diversity appear. The already present in the Cambrian brachiopods make by a large Radiation; many groups appearing for the first time. In the Ordovician, the actual radiation of cephalopods ( Cephalopoda ), which had emerged already in the uppermost Cambrian began with simple shapes. They are among the largest predators of the Ordovician, with case lengths up to 10 m or more (eg Ord Endocerida ). In the group of echinoderms ( Echinodermata ) enter the sea urchins ( Echinoidea ), the sea cucumbers ( Holothuroidea ), the starfish ( Asteroidea ) and brittle stars ( Ophiuroidea ) for the first time on. Furthermore the specific rapid radiation of crinoids ( Crinoida ) emphasized. The strange group of Carpo Idea makes its appearance for the first time. The trilobites diversify; among them are now nektonische forms with large, highly developed compound eyes, but also (secondary) blind forms, which probably inhabited deeper water. Among the jawless vertebrate relatives ( Agnatha ), the Pteraspidomorphi developed. The conodonts also developed very rapidly. After the extinction of Archaeocyaths now formed the first corals, bryozoans and stromatoporoids reefs.

At the end of the Ordovician it came to a mass extinction. About 100 families of marine organisms died out. The cause was probably a combination of several factors: global cooling by the hirnantische glaciation, sea level lowering, the closure of the Iapetus and Others The biostratigraphic zone classification is mainly based on graptolites, trilobites, brachiopods and conodonts.

Development of Flora

Green algae were common in the upper Cambrian and Ordovician. Probably from the first simple land plants today's liverworts appeared during the Ordovician in the form of non-vascular mosses similar. Spores of this first land plants were found in the uppermost Ordovician sediments.

It is believed that arbuscular mycorrhizal fungi were among the first land-living fungi and have played a major role in the colonization of land by plants, by entering into a symbiosis with the plant and mineral nutrients made ​​available to them. Such 460 million years old fossilized hyphae and spores were found in Wisconsin.

The Ordovician in Central Europe

By the sea level highs much of the land masses were flooded and it came to the deposition of shallow marine sediments. Characteristic of the Ordovician are calcium deposits, including large parts of present-day Scandinavia (eg Sweden) and the Baltic states ( Estonia, for example ). In many areas, the source rocks were deposited from oil and natural gas, such as the Estonian Kukersit. In Germany, there are mainly clay deposits ( shale ) from the Ordovician. Especially in Thuringia these sedimentary rocks contain fossils. A special feature of them is the leather shale dar. He (often quartzites ) contains interpreted as dropstones clasts that contain fossils while the surrounding shale least as fossilarm applies. They are an important indication of the time still in the South Pole near distributors Armorica group of small continents, which were later merged with Baltica and today form the base of Central Europe.