End–Ordovician extinction event

The End–Ordovician extinction event is the third-largest extinction event of the Phanerozoic eon. The Ordovician period followed the Cambrian and was followed by the Silurian. There were no living things on the land except for bacteria and perhaps some single-celled algae. The biota was almost entirely marine.
The extinction came in two steps, at the start and the finish of the "Hirnantian" stage, which was the last stage of the Ordovician. 
Basic mechanism: climate changes from very warm to very cold and back to very warm. Changes in ocean circulation were the results of the climate changes. Both benthic (ocean bottom) and pelagic fauna were faced with conditions they were unable to cope with.
More than 100 invertebrate families became extinct in the End–Ordovician extinction event, and a total of almost half the genera. The brachiopods and bryozoans were decimated, along with many of the trilobite, conodont and graptolite families. 
The cause was probably the rise and erosion of the Appalachian Mountains. The rise put much CO2 into the atmosphere, and the erosion took it out.