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# Ordovician – Silurian extinction event ( End Ordovician or O-S ): 450 – 440 Ma at the Ordovician-Silurian transition.

Two events occurred that killed off 27 % of all families and 57 % of all genera.

Together they are ranked by many scientists as the second largest of the five major extinctions in Earth's history in terms of percentage of genera that went extinct.