The Silurian is a geologic period and system that extends from the end of the Ordovician Period, at million years ago (mya), to the beginning of the Devonian Period. As with other geologic periods, the rock beds that define the period's start and end are well identified, but the exact dates are uncertain by several million years. The base of the Silurian is set at a major extinction event when 60% of marine species were wiped out. See Ordovician-Silurian extinction events.