Ordovician

The Ordovician (/ɔːrd.əˈvɪʃ.ən/) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.2 million years from the end of the Cambrian Period 485.4 million years ago (Mya) to the start of the Silurian Period 443.8 Mya.The Upper Ordovician edrioasteroid Cystaster stellatus on a cobble from the Kope Formation in northern Kentucky. In the background is the cyclostome bryozoan Corynotrypa.Fossil Mountain, west-central Utah; Middle Ordovician fossiliferous shales and limestones in the lower half.Outcrop of Upper Ordovician rubbly limestone and shale, southern Indiana; College of Wooster students.Outcrop of Upper Ordovician limestone and minor shale, central Tennessee; College of Wooster students.Trypanites borings in an Ordovician hardground, southeastern Indiana.Petroxestes borings in an Ordovician hardground, southern Ohio.Outcrop of Ordovician kukersite oil shale, northern Estonia.Bryozoan fossils in Ordovician kukersite oil shale, northern Estonia.Brachiopods and bryozoans in an Ordovician limestone, southern Minnesota.Vinlandostrophia ponderosa, Maysvillian (Upper Ordovician) near Madison, Indiana. Scale bar is 5.0 mm.The Ordovician cystoid Echinosphaerites (an extinct echinoderm) from northeastern Estonia; approximately 5 cm in diameter.Prasopora, a trepostome bryozoan from the Ordovician of Iowa.An Ordovician strophomenid brachiopod with encrusting inarticulate brachiopods and a bryozoan.The heliolitid coral Protaraea richmondensis encrusting a gastropod; Cincinnatian (Upper Ordovician) of southeastern Indiana.Zygospira modesta, atrypid brachiopods, preserved in their original positions on a trepostome bryozoan; Cincinnatian (Upper Ordovician) of southeastern Indiana.Graptolites (Amplexograptus) from the Ordovician near Caney Springs, Tennessee. The Ordovician (/ɔːrd.əˈvɪʃ.ən/) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.2 million years from the end of the Cambrian Period 485.4 million years ago (Mya) to the start of the Silurian Period 443.8 Mya. The Ordovician, named after the Celtic tribe of the Ordovices, was defined by Charles Lapworth in 1879 to resolve a dispute between followers of Adam Sedgwick and Roderick Murchison, who were placing the same rock beds in northern Wales into the Cambrian and Silurian systems, respectively. Lapworth recognized that the fossil fauna in the disputed strata were different from those of either the Cambrian or the Silurian systems, and placed them in a system of their own. The Ordovician received international approval in 1960 (forty years after Lapworth's death), when it was adopted as an official period of the Paleozoic Era by the International Geological Congress. Life continued to flourish during the Ordovician as it did in the earlier Cambrian period, although the end of the period was marked by the Ordovician–Silurian extinction events. Invertebrates, namely molluscs and arthropods, dominated the oceans. The Great Ordovician Biodiversification Event considerably increased the diversity of life. Fish, the world's first true vertebrates, continued to evolve, and those with jaws may have first appeared late in the period. Life had yet to diversify on land. About 100 times as many meteorites struck the Earth per year during the Ordovician compared with today. The Ordovician Period began with a major extinction called the Cambrian–Ordovician extinction event, about 485.4 Mya (million years ago). It lasted for about 42 million years and ended with the Ordovician–Silurian extinction events, about 443.8 Mya (ICS, 2004) which wiped out 60% of marine genera. The dates given are recent radiometric dates and vary slightly from those found in other sources. This second period of the Paleozoic era created abundant fossils that became major petroleum and gas reservoirs. The boundary chosen for the beginning of both the Ordovician Period and the Tremadocian stage is highly significant. It correlates well with the occurrence of widespread graptolite, conodont, and trilobite species. The base (start) of the Tremadocian allows scientists to relate these species not only to each other, but also to species that occur with them in other areas. This makes it easier to place many more species in time relative to the beginning of the Ordovician Period. A number of regional terms have been used to subdivide the Ordovician Period. In 2008, the ICS erected a formal international system of subdivisions. There exist Baltoscandic, British, Siberian, North American, Australian, Chinese Mediterranean and North-Gondwanan regional stratigraphic schemes. The Ordovician Period in Britain was traditionally broken into Early (Tremadocian and Arenig), Middle (Llanvirn (subdivided into Abereiddian and Llandeilian) and Llandeilo) and Late (Caradoc and Ashgill) epochs. The corresponding rocks of the Ordovician System are referred to as coming from the Lower, Middle, or Upper part of the column. The faunal stages (subdivisions of epochs) from youngest to oldest are: Late Ordovician Middle Ordovician