Simplify the following text for a Grade 7 native English-speaking person.
Original: The base of the Silurian is set at a major extinction when 60 % of marine species were wiped out , the Ordovician-Silurian extinction events .
Simplified: 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 .