Ordovician-Silurian mass extinction
The third largest extinction in Earth's history, the Ordovician-Silurian mass extinction had two peak dying times separated by hundreds of thousands of years. During the Ordovician, most life was in the sea, so it was sea creatures such as trilobites, brachiopods and graptolites that were drastically reduced in number. In all, some 85% of sea life was wiped out. An ice age has been blamed for the extinctions - a huge ice sheet in the southern hemisphere caused climate change and a fall in sea level, and messed with the chemistry of the oceans.