Silurian

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< Ordovician | S ilur | Devonian >
443.4-419.2 million years ago
Atmospheric O 2 share
(average over period) 		approx. 14 vol .-%
(70% of today's level)
Atmospheric CO 2 share
(average over period) 		approx. 4500 ppm
(12 times today's level)
Floor temperature (average over period) approx. 17 ° C
(3 ° C above today's level)
system series step ≈ age ( mya )
higher higher higher younger
Silurian Pridolium 419.2
⬍
423
Ludlow Ludfordium 423
⬍
425.6
Gorstium 425.6
⬍
427.4
Wenlock Homerium 427.4
⬍
430.5
Sheinwoodium 430.5
⬍
433.4
Llandovery Telychium 433.4
⬍
438.5
Aeronium 438.5
⬍
440.8
Rhuddanium 440.8
⬍
443.4
deeper deeper deeper older

The Silurian is the third chronostratigraphic system (or period in geochronology ) of the Paleozoic Era in the history of the earth . This period 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 Devonian system .

History and naming

The name comes from the Silurians , a Celtic tribe in South Wales , and was coined by Roderick Murchison in 1833. In the past, the Silurian was also called Gotlandium , as rock layers from this system are exemplarily represented on the Baltic island of Gotland .

Silurian reef complex on Gotland

Roderick Murchison and his friend Adam Sedgwick were already working on the Paleozoic deposits in Wales in the 1830s. Since Sedgwick had named the strata he studied after the Latin name of Wales ( Cambria ) Cambrian , Murchison did the same and named the younger strata he described as Silurian after the Celtic-Welsh tribe. Together, they wrote 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 layers in England and Wales successive), which appeared in 1835. Later, Charles Lapworth introduced the term Ordovician for those rock layers whose membership of one of the two layer sequences could not be agreed upon , also named after an ancient Welsh tribe.

Definition and GSSP

Simplified representation of the formation of Laurussia from the Ordovician to the Devonian

The International Union of Geological Sciences (IUGS) determined the first occurrence of the graptolite species Parakidograptus acuminatus and Akidograptus ascensus as the basis of the Silurian ; the upper limit (= lower limit of the Devonian) is the first appearance of the graptolite species Monograptus uniformis . The GSSP ( G lobal S tratotype S ection and P oint = Global calibration point for stratotypes ) Silurian is a profile on Dob's Linn in Moffat in Scotland .

Subdivision of the Silurian

The chronostratigraphic system of the Silurian is divided into four series , which in turn are subdivided into a total of eight levels , with the Pridolium series also corresponding to one level.

Paleogeography

The arrangement of the continents, which is typical for the Cambrian and Ordovician, changed fundamentally in the Silurian. Already during the Ordovician, Laurentia and Baltica (including the microcontinent Avalonia, which merged with Baltica in the Upper Ordovician ) were moving towards each other , closing the Iapetus Ocean. In the lower Silurian it came to the collision of the two major continental plates and the formation of the Caledonian fold belt . When Laurentia and Baltica merged, the Laurussia continent was created . The Rhine Ocean between Gondwana in the south and Baltica and Laurentia (or after their collision Laurussia) in the north reached its maximum width around the Silurian. In the Upper Silurian, the Hun super terran broke off the northern edge of Gondwana and drifted north towards Laurussia. The Rheic Ocean between the Hun superterran and Laurussia was subducted under the Hun superterran . Between the Hun super terran and Gondwana, the Palaeotethys ocean began to open.

climate

Live reconstructions of the fish fauna from the Ludlow of China: front: bony fish Sparalepis ; middle: 2 individuals of the placoderma Entelognathus ; back: 2 individuals of the bony fish Megamastax , the largest known vertebrate of the Silurian. In between numerous conodonts.

In the course of the Silurian, the oxygen concentration reached values ​​of 14 percent for the first time, and the carbon dioxide concentration fell to below 4000 ppm towards the end of the 24.2 million year epoch . After the Andean-Sahara Ice Age subsided, a warm, temperate climate prevailed with a global average of around 17 ° C, with the meridional temperature gradient (the temperature gradient from the equator to the polar regions) in both the Silurian and the following Devonian regions being flatter than it is today . Reef formation was common at low latitudes. Since, apart from a few short-term and spatially limited glacier formations, the earth was almost free of ice, the sea level remained at a high level and the continental margins were flooded by extensive shallow seas.

Community on a Silurian reef (reconstruction of life)

With a focus on the Wenlock series 433.4 to 427.4 million years ago, there were several extinction events . The marine life forms of the conodonts and various plankton groups such as the graptolites were particularly affected . In the latter, the rate of extinction rose gradually up to 95 percent, before biodiversity increased again over longer periods of time. The main causes of these biological crises are predominantly plate tectonic activities in connection with extremely intense volcanism. The volcanic outgassing caused chemical and climatic anomalies both in the atmosphere and in the oceans and subsequently probably led to a disruption of the short and long-term carbon cycle . At the end of the Silurian, three minor extinction events occurred, including the so-called Lau event , which began in the Ludfordium .

Development of the fauna

Life community on the sea ​​floor (living reconstruction)

The Ordovician-Silurian border was a decisive turning point. The first jaw-bearing vertebrates (Gnathostomata) appeared. The Placodermi appeared in the Lower Silurian and developed a considerable diversity during the Silurian. The first fossil remains of the bony fish (Osteichthyes) have been found in the Obersilur . They lived together with huge, up to two meters long sea ​​scorpions in the shallow sea. These had already developed in the Ordovician, but had the greatest diversity in the Silurian and Devonian. The corals, represented by the two large groups of Tabulata and Rugosa , formed larger reef structures (e.g. Gotland ). Within the echinoderms (Echinodermata) the bud rays (blastoidea) appeared for the first time. The classes Eocrinoidea and Paracrinoidea became extinct . In the tribe of the Armfüßer (Brachiopoda) the order Trimerellida died out at the end of the Silurian.

Development of flora

The land plants continued to develop and spread. The first vascular plants appeared in the Middle Silurian with Cooksonia on Laurussia and Baragwanathia on Gondwana . Psilophyton is an original land plant with xylem and phloem , but not yet differentiated into root, stem and leaves . They carried out photosynthesis over the entire surface, and the stomata were also distributed over the entire surface. It reproduced via spores and is at the base of the ancient ferns (Psilophytopsida), which, however, had their actual development in the Devonian . The Rhyniophyta and simple club moss plants (Lycopodiophyta) also have their origins in the Silurian. Lichen are also first detected in the Silurian.

The Silurian in Central Europe

Silurian slate (Nossen-Wilsdruffer Schiefergebirge)

Dark, bituminous clay stones (" graptolite slates") are very characteristic of the Silurian in large parts of Central Europe . Silica and alum shale are also found subordinate. In Bohemia , the Upper Silurian is represented by dark, shallow marine limestone. In the Carnic Alps , the Silurian is also limestone. There and in Bohemia there are numerous volcanic layers.

literature

  • Ivo Chlupáč , Z. Kukal: The boundary stratotype at Klonk. The Silurian-Devonian Boundary. IUGS Series, A5, Berlin 1977, pp. 96-109, ISSN  0374-8480
  • L. Robin M. Cocks : The Ordovician-Silurian Boundary. In: Episodes , 8 (2), Beijing 1985, pp. 98-100, ISSN  0705-3797 .
  • Donald G. Mikulic, Derek EG Briggs, Joanne Kluessendorf: A new exceptionally preserved biota from the Lower Silurian of Wisconsin, USA. In: Philosophical Transactions of the Royal Society of London , 311B, London 1985, pp. 75-86.
  • L. Robin M. Cocks, Trond H. Torsvik: European geography in a global context from the Vendian to the end of the Palaeozoic. In: DG Gee, RA Stephenson (Ed.): European Lithosphere Dynamics. In: Geological Society London Memoirs , 32, London 2006, pp. 83-95, ISSN  0435-4052
  • Gérard M. Stampfli, Jürgen F. von Raumer, Gilles D. Borel: Paleozoic evolution of pre-Variscan terranes: From Gondwana to the Variscan collision. In: Geological Society of America Special Paper , 364, Boulder 2002, pp. 263-280, PDF
  • Roland Walter: Geological history, the formation of the continents and oceans. 5th edition de Gruyter, Berlin / New York 2003, ISBN 3-11-017697-1 , 325 pp.

Web links

Commons : Silurian  - collection of images, videos and audio files

Individual evidence

  1. ↑ Oxygen content-1000mj
  2. ↑ Phanerozoic Carbon Dioxide
  3. ↑ All palaeotemps
  4. ↑ Bradley D. Cramer, Daniel J. Condon, Ulf Söderlund, Carly Marshall, Graham J. Worton, Alan T. Thomas, Mikael Calner, David C. Ray, Vincent Perrier, Ian Boomer, P. Jonathan Patchett, Lennart Jeppsson: U -Pb (zircon) age constraints on the timing and duration of Wenlock (Silurian) paleocommunity collapse and recovery during the “Big Crisis” . (PDF) In: Geological Society of America (Bulletin) . 124, No. 11-12, October 2012, pp. 1841-1857. doi : 10.1130 / B30642.1 .