The two Ordovician-Silurian Extinction events, taken together, were the second largest of the major extinction events that have occurred in the Earth's history. The only larger one was the Permian-Triassic extinction event.

They occurred approximately 450-440 million years ago, and mark the boundary between the Ordovician Period and the following Silurian one. At that time all complex organisms lived in the sea and around 100 marine families became extinct, covering about 85% of species of fauna. The brachiopods and bryozoans were particularly affected, along with many of the trilobite, conodont and graptolite families.

The most commonly accepted theory is that the first event was caused by the onset of a long ice age, which affected the shallow seas where most organisms lived. As the supercontinent Gondwana drifted over the South Pole, glaciers formed on it which caused sea levels worldwide to drop. This was accompanied by changing currents, including increased deep-ocean currents which caused deep ocean oxygenation and brought up nutrients or toxic material from them. Surviving species evolved to cope with the changed conditions and to fill the ecological niches left by the ones which had become extinct. The second event occurred when the continent drifted north again, melting the glaciers and causing the sea level to rise once more.

It has also been suggested that the first extinction was caused by a gamma ray burst originating from an exploding star within 6,000 light years of Earth (i.e. most probably within the Milky Way). A ten-second burst would have stripped the Earth's atmosphere of half of its ozone almost immediately, causing surface-dwelling organisms to be exposed to high levels of ultraviolet radiation. This would have killed many species and caused a drop in temperatures. [1]