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proterozoic
The Proterozoic is a geological eon representing the time just before the proliferation of complex life on Earth. The name Proterozoic comes from Greek and means "earlier life". The Proterozoic Eon extended from to (million years ago), and is the most recent part of the Precambrian. It is subdivided into three geologic eras (from oldest to youngest): the Paleoproterozoic, Mesoproterozoic, and Neoproterozoic.
The well-identified events of this eon were the transition to an oxygenated atmosphere during the Paleoproterozoic; several glaciations, including the hypothesized Snowball Earth during the Cryogenian period in the late Neoproterozoic; and the Ediacaran Period (635 to 542 Ma) which is characterized by the evolution of abundant soft-bodied multicellular organisms.
==The Proterozoic record==
The geologic record of the Proterozoic is much better than that for the preceding Archean. In contrast to the deep-water deposits of the Archean, the Proterozoic features many strata that were laid down in extensive shallow epicontinental seas; furthermore, many of these rocks are less metamorphosed than Archean-age ones, and plenty are unaltered. Study of these rocks shows that the eon continued the massive continental accretion that had begun late in the Archean, as well as featured the first definitive supercontinent cycles and wholly modern orogenic activity.〔Stanley, 315–18, 329–32〕
Evidence for the first known glaciations occurred during the Proterozoic; one began shortly after the beginning of the eon, while there were at least four during the Neoproterozoic, possibly climaxing with the hypothesized Snowball Earth of the Sturtian and Marinoan glaciations.〔Stanley, 320-1, 325〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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