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Stratigraphy of the Cambrian period currently has several schemes used for ordering geologic formations from the period. The International Commission on Stratigraphy−ICS scheme has set a stratotype section for the base of the Cambrian, dated quite accurately to million years ago. Russian and Chinese scientists have developed a different scheme. Stratigraphy relates to the order of rock units, without referring to their absolute ages (which is chronology). However, because fossils — which are traditionally the cornerstone of stratigraphy — are relatively rare in the Cambrian period, chronology has a significant part to play. If an absolute age can be obtained with a high degree of accuracy for different strata, their relative age can also be established. The base of the Cambrian is officially defined as the first appearance of a certain trace fossil, ''Treptichnus pedum''. However, this fossil also appears in older rocks than the locality which officially marks the start of the period. However, there is a period of biological change which makes this time period a good one to demarkate the Cambrian from the earlier Phanerozoic eon's periods and the Precambrian supereon. Consensus holds that fossils of the Ediacara biota disappear here, as do some shelly fossils and acritarchs; and a new small shelly fossil biota emerges. ==Biological changes at the base of the Cambrian== The opening of the Cambrian period is marked by a number of biological changes, including the extinction of the Ediacara biota, the preponderance of armoured organisms (e.g. the small shelly fossils), and a "widening of the behavioural repertoire"〔S. Conway Morris, 2001〕 indicated primarily by an increase in vertical burrowing, first for protection and later for feeding - the Cambrian substrate revolution. This combination of factors marks the end of the slow-moving, extinction-proof Precambrian period, and the start of a more complex food web with hugely different dynamics. However, since there are a large number of inter-related biological factors, defining a single stratum to define the base of this period is impossible - the change did not happen over-night, but over a prolonged period. The sequence of events leading to the Cambrian revolution in fact begun in the Ediacaran period, and culminated in the middle of the Cambrian;〔 defining a sharp base to a gradual biological process is impossible, but it is necessary to have a globally recognised base to the period to allow scientists across the globe to know what each other are referring to and to converse meaningfully. 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「Stratigraphy of the Cambrian」の詳細全文を読む スポンサード リンク
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