Cambrian

The Cambrian is a period of Earth's history, which roughly corresponds to the period millions of years ago about 541 to 485.4. During this period originated in the so-called Cambrian explosion due to changing environmental conditions in the sea almost all of today's animal phyla.

System

In the Cambrian is the lowest chronostratigraphic system and the oldest period of the Paleozoic geochronological and thus the Phanerozoic in the Earth's history. Among them or in front of it is the Ärathem or the era of the Neoproterozoic ( Proterozoic Eon ) with the Ediacaran. The system or after the period of the Ordovician Additionally follows. From the layers which are older than the Cambrian, very few fossils have become known. The entire period of the formation of the Earth was created around 4.56 billion years ago to the development of animal life in the Cambrian is referred to in the older literature as Precambrian.

History and naming

The name of the Cambrian period for this system was proposed by Adam Sedgwick in 1835 by the Latin name of Wales ( Cambria ), because there are layers of the Cambrian open-minded.

The beginning of the Cambrian

For a long time it was assumed the beginning of the Cambrian at about 600 million years before the present to, with the seemingly first appearance of fossils. Only recently fossils were found and studied in older layers. Just 20 years ago the beginning of the Cambrian was fixed before 590-570 million years ago, with the onset of trilobites and Archaeocyaths (first sponges with calcareous skeleton ). This was postponed in recent decades ever closer towards the present by radiometric methods. 542 million years ago there is a turning point in the global distribution of carbon isotope C -13, which is accompanied by a fundamental change in the fossil fauna. Through studies of zircons in volcanic ash layers from the early Cambrian period in Oman the date of this C-13 anomaly could be determined with a precision of 0.3 million years ago. He is considered now as the beginning of the Cambrian, and thus the beginning of the geological era ( era ) Paleozoic.

End of the Cambrian

In the first half of the 19th century, the layers that lie directly on the recent layers of the Cambrian were still referred to as the Silurian. After problems with the demarcation was coined later for the basal layers of the Silurian in its original scope of the term Ordovician, this was sandwiched between the Cambrian and Silurian. In the older literature, therefore, the system is missing or the Ordovician, layers that are now attributed to the Ordovician, are referred to as Untersilur.

Definition and GSSP

The beginning of the Cambrian, and thus the Phanerozoic (and also the Terreneuvium Series and Fortunium stage of the Cambrian period ) was defined by the International Union of Geological Sciences ( IUGS ​​) with the first appearance of the trace fossil Trichophycus pedum. In addition, the limit is also very close to a negative carbon isotope anomaly. The upper limit (and thus also the beginning of the Ordovician ) is the first appearance of the conodont species Iapetognathus fluctivagus, which in turn lies just above the base of the Cordylodus lindstromi - conodont zone. This limit is only slightly below the first appearances of planktonic graptolites. For global reference profile ( GSSP ) for the base of the Cambrian was elected in 1992 a profile in the Chapel Iceland lineup at Fortune Head on the Burin Peninsula, Newfoundland ( Canada).

Subdivision of the Cambrian

The Cambrian is divided according to the current state of chronostratigraphy in four series with a total of 10 levels, but they are each not yet formally named in some cases. The 2009 level is as follows:

• System: Cambrian ( 541 to 485.4 mya ) Series: Furongium ( 497 to 485.4 mya ) Level: 10 Level Cambrian ( 489.5 to 485.4 mya )
• Level: Jiangshanium ( 494 to 489.5 mya )
• Level: Paibium ( 497-494 mya )

• Level: Guzhangium ( 500.5 to 497 mya )
• Level: Drumium ( 504.5 to 500.5 mya )
• Level: 5 Level Cambrian ( 509 to 504.5 mya )

• Level: 4 Level Cambrian ( 514-509 mya )
• Level: 3 Level Cambrian ( 521-514 mya )

• Level: 2 Level Cambrian ( 529-521 mya )
• Level: Fortunium ( 541-529 mya )

Paleogeography

At the beginning of the Cambrian there was a great southern continent of Gondwana, which reached with its northern reaches across the equator into northern latitudes. At this continent were not only the classic Gondwana continents (Africa, South America, India, Madagascar, Australia, Antarctica, Saudi Arabia and others), but also some smaller blocks, which were later welded to the northern continents, such as the small continent Avalonia (parts of Central and Western Europe), the Armorica -Terra group (parts of Western and Southern Europe ), the Tarim block, the Sino - Korean craton and the Yangtze craton. This large continent in the South were offset by three smaller continents. Laurentia ( of North America and Greenland), Baltica (Northern Europe) and Sibiria ( Siberia) were all slightly south of the equator. Laurentia was separated from Baltica and Gondwana by the Iapetus Ocean. Between Baltica and Gondwana upstream Avalonia was the Tornquist Ocean. Sibiria was separated by the Aegir Ocean of Baltica. Isolated from these continents was also a small continent Kasachstania, which was welded to Sibiria in the Carboniferous. The South Pole was located in the Lower Cambrian in present-day northern South America. He shifted to the end of the Cambrian to North Africa or Gondwana migrated accordingly over the South Pole time. The North Pole was at the time of the Cambrian in the sea.

Climate

At the beginning of the Cambrian global warming seems to have occurred. The sea level rose during the Cambrian considerably. The oxygen concentration in the atmosphere was lower at the beginning of the Cambrian than it is today, but had to put on a bit from the Cambrian and Precambrian outgoing rose during the Cambrian further slightly. The CO2 concentration increased during the Cambrian sharply and reached at the Cambrian / Ordovician boundary an absolute peak, which has not been achieved throughout the Phanerozoic.

Development of the fauna

The beginning of the Cambrian is characterized by the so-called " Cambrian explosion ," in which arose in a geologically very short period of very many multicellular animal or groups appear in the fossil record, their basic construction plans have been partially preserved. The beginning of the Cambrian marked thus, the development of the animal kingdom a very significant turning point in the history of the earth, with which also the Äonothem the Phanerozoic began that great geological section, in which the living world as we know it today developed.

With the exception of bryozoans ( Bryozoa ) were in the Cambrian already almost all modern animal phyla available: Sponges ( Porifera ), Cnidarians ( Cnidaria ), Arthropods ( Arthropoda ), brachiopods ( Brachiopoda ), molluscs ( Mollusca ), echinoderms ( Echinodermata ) and other smaller tribes of invertebrates, as well as the precursor groups of vertebrates. Many species developed the first hard skeletons and housing. Which is on the one hand for protection against the said first large predators also occurred at this time, on the other hand by the great deal of calcium carbonate. By a change in the chemical composition of the sea water The occurrence of cases and skeletons of calcium carbonate, which, of course, a much better Fossilisationspotenzial than merely soft tissues makes, explained why the Cambrian suddenly so many animal phyla occur over whose ancestors are unknown. Presumably, the splitting ( Radiation) must be moved far back into the Ediacaran of multicellular animals ( metazoans ).

As index fossils for biostratigraphic subdivision of the Cambrian are used:

• Trilobites
• Archaeocyaths
• Brachiopods

The well -counting of the sponges Archaeocyaths built the first major reefs in the earth's history. They died at the beginning of Oberkambriums again.

Development of Flora

From the Cambrian fauna only marine planktonic algae are known. The country was not yet colonized by plants.

The Cambrian in Central Europe

In Central Europe, there are very few outcrops or areas where rocks of the Cambrian come to the surface. It is covered in most areas of thick younger sedimentary layers and / or been metamorphosed and also for later orogenies. Europe is made up of different tectonic plates ( Laurentia, Baltica, Avalonia and Armorica - terranes ) that were very far apart at the time of the Cambrian in some cases. They were together until later orogenies in this position. According multiform are the facies and the Fauneninhalt the Cambrian strata in Central Europe.

In Germany Cambrian age are in the following regions rocks been found: Black Forest, Spessart, North Rhine -Westphalia, Lower Saxony, North Thuringia, Thuringian- Franconian Slate Mountains, Fichtelgebirge, the Bavarian Forest, Upper Palatinate Forest, the Erzgebirge, Vogtland, Lausitz, inter alia, as well as in some holes in northern Germany, where particularly the bore " Adlergrund " in the Baltic Sea is important. While the said other information areas are all part of Avalonia and Armorica -Terra Group, so still belonged to Gondwana during the Cambrian period, the area of the hole Adlergrund was in the Cambrian to Baltica.

In addition, Cambrian sediment in northern Germany are widespread. They come from the Scandinavian deposits, but also contain fossils that have not been found in the area of ​​origin, such as Xenusion.

Fossil deposits

From the Burgess Shale in the Rocky Mountains of Canada many well-preserved fossils from the Middle Cambrian are known, especially arthropods, annelids, Onychophora, Priapuliden next trilobites, sponges and fossils that can be assigned to any of today's tribes. Even older is the famous Chengjiang Faunengemeinschaft in Maotianshan Shale in China (Province of Yunnan ). These fossil site is famous for its excellent preservation of soft tissues. Other notable Cambrian fossil sites are the Orsten. Orsten are calcareous nodules, which are incorporated in alum. In these calcareous nodules Chitinskelette phosphatised were at an early stage of diagenesis and preserved in three dimensions. With weak acid, these excellently preserved Chitinskelette of Cambrian arthropods and their larval stages could be dissolved out of the rock. The term Orsten comes from Sweden, where two such fossil sites are known. Meanwhile, a " Orsten " fossil site was discovered in the Cambrian of Australia.