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Ordovician Silurian Extinction

The Ordovician Silurian Extinction was actually two extinction in close proximity that took place 440-450 million years ago, killing off over 60% of all marine species. Possible causes:

Weathering: heavy weathering of the Appalachian Mountains out-weighed volcanism, resulting in abnormally large CO2 sequestering, which lead to the development of a rapid ice age.

Gamma Ray Burst: this is a speculative hypothesis, but a gamma ray burst from a hypernova within 6,000 light years could have irradiated Earth. Within 10 seconds of the burst, half of the ozone layer would have been destroyed, bathing the planet in ultraviolet radiation.