The Silurian is a geologic period and system that extends from the end of the Ordovician
Period, about 443.7 ± 1.5 million years ago (mya), to the beginning of the Devonian
Period, about 416.0 ± 2.8 mya. 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 eciesSP were wiped out.