This extinction happened about 439 million years ago because of a drop in sea levels as glaciers formed. An ice age has been blamed for the cause of the extinctions. A large ice sheet in the southern hemisphere caused climate change and a decrease in sea levels. A combination of this lowering of sea level caused a reduction in ecospace on continental shelves, and the cooling caused by the glaciation itself are some of the big factors for the Ordovician mass extinction. This drop in temperature caused more than 60% of marine invertebrates to die along with two-thirds of all brachiopod and bryozoan families.
Number 2: The Devonian extinction
The extinction happened about 364 million years ago.