Silurian

The Silurian is in Earth's history, the third chronostratigraphic system ( or period in geochronology ) of the Paleozoic. The period of the Silurian lasted from about 443.4 million years ago to about 419.2 million years ago. The Silurian follows the Ordovician and is replaced by the system of Devon.

History and naming

The name comes from the Silurern, a Celtic tribe in South Wales, and was coined by Roderick Murchison in 1833. Previously, the Silurian was also called Gotlandium because rock layers of this system on the Baltic island of Gotland are exemplified.

Sir Roderick Murchison worked together with his friend Adam Sedgwick already in the 1830s the Paleozoic deposits in Wales. As Sedgwick had he investigated layers after the old name for Wales ( Cambria ) called Cambrian, he did the same and named the younger layers, which were described by him, according to a Celtic- Welsh tribe as Silurian. Together authored both the work On the Silurian and Cambrian Systems, Exhibiting the Order in Which the Older Sedimentary Strata Succeed each other in England and Wales (On the Silurian and Cambrian periods. Representation of the arrangement, in which the older sedimentary strata in England and Wales follow each other ), which appeared in 1835. Later, by Charles Lapworth for those rock layers, whose belonging to one of the two layer sequences could not agree to the term introduced Ordovician, also named after an ancient Welsh tribe.

Definition and GSSP

As the base of the Silurian of the International Union of Geological Sciences ( IUGS ​​), the first appearance of the graptolite species Parakidograptus acuminatus and Akidograptus ascensus was determined; the upper limit ( = lower limit of the Devonian ) is the first appearance of the graptolite - type Monograptus uniformis. The GSSP (Global Stratotype Section and Point = Global calibration point for stratotypes ) of the Silurian is a profile at Dob's Linn, at Moffat in Scotland.

Subdivision of the Silurian

The chronostratigraphic system of the Silurian is divided into four series, which are in turn subdivided into a total of eight stages, the Pridolium series also corresponds to a stage.

• System: Silurian ( 443.4 to 419.2 mya ) Series: Pridolium ( 423 to 419.2 mya ) (not divided into more stages )
• Series: Ludlow ( 427.4 to 423 mya ) Level: Ludfordium ( 425.6 to 423 mya )
• Level: Gorstium ( 427.4 to 425.6 mya )

• Level: Homerium ( 430.5 to 427.4 mya )
• Level: Sheinwoodium ( 433.4 to 430.5 mya )

• Level: Telychium ( 438.5 to 433.4 mya )
• Level: Aeronium ( 440.8 to 438.5 mya )
• Level: Rhuddanium ( 443.4 to 440.8 mya )

Paleogeography

The typical Cambrian and Ordovician arrangement of the continents changed fundamentally in the Silurian. During the Ordovician Laurentia and Baltica is moving (including in Oberordovicium merged with Baltica microcontinent Avalonia ) under subduction of the Iapetus Ocean, towards each other. In the Lower Silurian it came to the collision of the two great continental plates and the formation of the Caledonian fold belt. With the amalgamation of Laurentia and Baltica Laurussia a new major continent was formed. The Rheische ocean between Gondwana in the south and Baltica and Laurentia (or after the collision of the two continents Laurussia ) in the north reached its maximum width about in the Silurian. In the Silurian the Hun - Super Terran from the northern edge of Gondwana broke off and drifted north on Laurussia to. The Rheische ocean between the Hun - Super Terran and Laurussia was subducted under the Hun - Super Terran. Between the Hun - Super Terran and Gondwana, the Palaeotethys began to open.

Climate

The climate was globally considered quite warm, also influenced by the high proportion of atmospheric carbon dioxide and the associated greenhouse effect. Nevertheless, can be found from this almost 28 million years lasting era also references to temporary domestic icing. In the low latitudes, there was widespread in the formation of reefs. The water level was relatively high; which led to the formation of shallow seas on each continent.

Development of the fauna

The Ordovician - Silurian boundary was a dramatic break. The first jawed vertebrates ( gnathostomata ) occurred. In Untersilur the placoderms, the already developed a considerable diversity during the Silurian published. In the Silurian the first fossil remains of bony fish ( Osteichthyes ) have been demonstrated. They lived together with huge, up to two meters long Seeskorpionen in shallow sea. This had already developed in Ordovician, Silurian and Devonian but had the greatest diversity. The corals, operates the two major clades of tabulata and Rugosa, formed by larger reef structures (eg Gotland ). Within the echinoderms ( Echinodermata ) the buds radiator ( Blastoidea ) occurred for the first time. The classes Eocrinoidea and Paracrinoidea died out. The family of brachiopods ( Brachiopoda ) the order Trimerellida died out at the end of the Silurian. At the end of the Silurian three smaller extinction events are observed, including the so-called Lau event, which began in Ludfordium.

Development of Flora

The land plants continued to evolve and spread. The first vascular plants appeared in Mittelsilur with Sonia Cook on Laurussia and Baragwanathia on Gondwana. An original land plant with xylem and phloem, but still no differentiation in root, stem and leaves, is Psilophyton. It operated photosynthesis over the entire surface, and the stomata were distributed over the entire surface. They multiplied via spores and is at the base of Urfarne ( Psilophytopsida ), but had their actual development in Devon. The Rhyniophyta and easy Bärlapppflanzen ( Lycopodiophyta ) have also originated already in the Silurian. Lichens are also first detected in the Silurian.

The Silurian in central Europe

Very characteristic of the Silurian in large parts of Central Europe are dark, bituminous shales ( " graptolites - slate "). Subordinate also gravel and alum be found. In Bohemia the upper Silurian is represented by dark, shallow marine limestones. In the Carnic Alps the Silurian is also formed chalky. Here and also in Bohemia are on numerous volcanic layers.