What was the ordovician-silurian extinction?

It was the second largest extinction in terms of loss of life. The extinction was caused by the movement of Gondwana into the south polar region which led to glaciation and subsequent sea level fall. At this time all know life was confined to the oceans and 60% of marine invertebrates died. The event ended when Gondwana moved out of the south polar region, the glaciers melted, and sea level returned to normal.