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The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between and million years ago.〔(【引用サイトリンク】 title=International Chronostratigraphic Chart v.2015/01 )〕 It follows the Cambrian Period and is followed by the Silurian Period. 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 periods, respectively.〔Charles Lapworth (1879) ( "On the Tripartite Classification of the Lower Palaeozoic Rocks," ) ''Geological Magazine'', new series, 6 : 1-15. From pp. 13-14: "North Wales itself — at all events the whole of the great Bala district where Sedgwick first worked out the physical succession among the rocks of the intermediate or so-called ''Upper Cambrian'' or ''Lower Silurian'' system; and in all probability much of the Shelve and the Caradoc area, whence Murchison first published its distinctive fossils — lay within the territory of the Ordovices; … Here, then, have we the hint for the appropriate title for the central system of the Lower Palaeozoics. It should be called the Ordovician System, after this old British tribe."〕 Lapworth, recognizing that the fossil fauna in the disputed strata were different from those of either the Cambrian or the Silurian periods, realized that they should be placed in a period of their own. While recognition of the distinct Ordovician period was slow in the United Kingdom, other areas of the world accepted it quickly. It received international sanction in 1960, 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 Cambrian, although the end of the period was marked by a significant mass extinction. Invertebrates, namely molluscs and arthropods, dominated the oceans. 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. ==Dating: extinction events== The Ordovician Period started at a major extinction event called the Cambrian–Ordovician extinction events about Mya (million years ago), and lasted for about 42 million years. It ended with the Ordovician–Silurian extinction event, about Mya (ICS, 2004) that wiped out 60% of marine genera. The dates given are recent radiometric dates and vary slightly from those used in other sources. This second period of the Paleozoic era created abundant fossils and in some regions, major petroleum and gas reservoirs. The boundary chosen for the beginning of both the Ordovician Period and the Tremadocian stage is highly significant. Since 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「ordovician」の詳細全文を読む スポンサード リンク
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