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__NOTOC__ In stratigraphy, paleontology, geology, and geobiology an erathem is the total stratigraphic unit deposited during a certain corresponding span of time during an era in the geologic timescale. It can therefore be used as a chronostratigraphic unit of time which delineates a large span of years — less than an geological eon, but greater than its successively smaller and more refined subdivisions (geologic periods, epochs, and geologic ages). By 3,500 Million years ago (Mya) simple life had developed on earth (the oldest known microbial fossils in Australia are dated to this figure.〔name="Rockmans" /> The atmosphere was a mix of noxious and poisonous gases (Methane, Ammonia, Sulphur compounds, etc.〔— a so-called reducing atmosphere〔basis for the Miller–Urey experiment〕 lacking much free oxygen which was bound up in compounds). These simple organisms, Cyanobacteria ruled the still cooling earth for approximately a thousand million (over a billion) years〔 and gradually transformed the atmosphere to one containing free oxygen. These changes, along with tectonic activity left chemical trails (red bed formation, etc.) and other physical clues (magnetic orientation, layer formation factors) in the rock record, and it is these changes along with the later richer fossil record which specialists use to demarcate times early in planet earth's history in various disciplines. Erathems are not often used in practice. While they are subdivisions of eonothems and are themselves subdivided into systems, dating experts prefer the finer resolution of smaller spans of time when evaluating strata. Erathems have the same names as their corresponding eras. :The Phanerozoic eonothem can thus be divided into a :: Cenozoic, a Mesozoic and a Paleozoic erathem or matching era name. Similarly, the Proterozoic eonothem is divided youngest to oldest into the ::Neoproterozoic, Mesoproterozoic and Paleoproterozoic erathems, :and the Archean eon and eonothem are divided similarly into the ::Neoarchean, Mesoarchean, Paleoarchean and the Eoarchean, for which a lower (oldest) limit is undefined.〔 />〔F.M. Gradstein, J.G. Ogg, A.G. Smith, et al., "A Geologic Time Scale 2004", (2004; Cambridge University Press).〕 ==See also== 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「erathem」の詳細全文を読む スポンサード リンク
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