End Ordovician - 444 million years ago
This mass extinction (which may actually have been several extinction events) occurred at the boundary between the Ordovician and Silurian periods, 444 million years ago. At this point all multicellular organisms lived in the oceans; there was nothing yet on land.
Cause:		The most widely accepted scientific theory is that a long ice age began around this time, which was accompanied by a fall in CO2 in the atmosphere. This kind of change in CO2 would also have affected the oceans, particularly shallow oceans. As this was where most of the marine life lived, the change triggered a mass extinction.
Result:		Roughly 50% of living fauna went extinct.