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The Cambrian ( or ) is the first geological period of the Paleozoic Era, lasting from to million years ago (mya) and is succeeded by the Ordovician.〔(【引用サイトリンク】url=http://www.stratigraphy.org/ICSchart/ChronostratChart2012.pdf )〕 Its subdivisions, and indeed its base, are somewhat in flux. The period was established (as “Cambrian series”) by Adam Sedgwick,〔 who named it after Cambria, the Latinised form of ''Cymru'', the Welsh name for Wales, where Britain's Cambrian rocks are best exposed.〔Sedgwick and R. I. Murchison (1835) ("On the Silurian and Cambrian systems, exhibiting the order in which the older sedimentary strata succeed each other in England and Wales," ) Notices and Abstracts of Communications to the British Association for the Advancement of Science at the Dublin meeting, August 1835, pp. 59-61, in: ''Report of the Fifth Meeting of the British Association for the Advancement of Science; held in Dublin in 1835'' (1836). From p. 60: "Professor Sedgwick then described in descending order the groups of slate rocks, as they are seen in Wales and Cumberland. To the highest he gave the name of ''Upper Cambrian'' group. … To the next inferior group he gave the name of ''Middle Cambrian''. … The ''Lower Cambrian'' group occupies the S.W. coast of Cærnarvonshire, … " 〕 The Cambrian is unique in its unusually high proportion of lagerstätte sedimentary deposits. These are sites of exceptional preservation, where "soft" parts of organisms are preserved as well as their more resistant shells. This means that our understanding of the Cambrian biology surpasses that of some later periods. The Cambrian marked a profound change in life on Earth; prior to the Cambrian, the majority of living organisms on the whole were small, unicellular and simple; the Precambrian ''Charnia'' being exceptional. Complex, multicellular organisms gradually became more common in the millions of years immediately preceding the Cambrian, but it was not until this period that mineralized – hence readily fossilized – organisms became common. The rapid diversification of lifeforms in the Cambrian, known as the Cambrian explosion, produced the first representatives of all modern animal phyla. Phylogenetic analysis has supported the view that during the Cambrian radiation, metazoa (animals) evolved monophyletically from a single common ancestor: flagellated colonial protists similar to modern choanoflagellates. While diverse life forms prospered in the oceans, the land was comparatively barren – with nothing more complex than a microbial soil crust〔Schieber, 2007, pp. 53–71.〕 and a few molluscs that emerged to browse on the microbial biofilm Most of the continents were probably dry and rocky due to a lack of vegetation. Shallow seas flanked the margins of several continents created during the breakup of the supercontinent Pannotia. The seas were relatively warm, and polar ice was absent for much of the period. The United States Federal Geographic Data Committee uses a "barred capital C" character similar to the capital letter Ukrainian Ye to represent the Cambrian Period. The proper Unicode character is .〔(Unicode Character 'LATIN CAPITAL LETTER C WITH BAR' (U+A792) ). fileformat.info. Accessed 15 Jun 2015〕 == Stratigraphy == Despite the long recognition of its distinction from younger Ordovician rocks and older Precambrian rocks, it was not until 1994 that this time period was internationally ratified. The base of the Cambrian is defined on a complex assemblage of trace fossils known as the ''Treptichnus pedum'' assemblage.〔A. Knoll, M. Walter, G. Narbonne, and N. Christie-Blick (2004) "(The Ediacaran Period: A New Addition to the Geologic Time Scale. )" Submitted on Behalf of the Terminal Proterozoic Subcommission of the International Commission on Stratigraphy.〕 Nevertheless, the usage of ''Treptichnus pedum'', a reference ichnofossil for the lower boundary of the Cambrian, for the stratigraphic detection of this boundary is always risky because of occurrence of very similar trace fossils belonging to the Treptichnids group well below the ''T. pedum'' in Namibia, Spain and Newfoundland, and possibly, in the western USA. The stratigraphic range of ''T. pedum'' overlaps the range of the Ediacaran fossils in Namibia, and probably in Spain.〔M.A. Fedonkin, B.S. Sokolov, M.A. Semikhatov, N.M.Chumakov (2007). "(Vendian versus Ediacaran: priorities, contents, prospectives. )" In: edited by M. A. Semikhatov "(The Rise and Fall of the Vendian (Ediacaran) Biota. Origin of the Modern Biosphere. Transactions of the International Conference on the IGCP Project 493, August 20–31, 2007, Moscow. )" Moscow: GEOS.〕〔A. Ragozina, D. Dorjnamjaa, A. Krayushkin, E. Serezhnikova (2008). "(''Treptichnus pedum'' and the Vendian-Cambrian boundary )". 33 Intern. Geol. Congr. 6–14 August 2008, Oslo, Norway. Abstracts. Section HPF 07 Rise and fall of the Ediacaran (Vendian) biota. P. 183.〕 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「Cambrian」の詳細全文を読む スポンサード リンク
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