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Geologic TimeScaleGeologists divide the earth´s history into units - in decreasing order of length, the units are eons, eras, periods, and epochs. Eras are important time units, and are marked by major extinctions. But when you date rocks or fossils, you usually just need to refer to the period, which is a subdivision of the era. Geologic periods are marked by the entrance or exit of certain key species (for example, the Ordovician is the period when fish first appeared.
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