Permian Triassic Extinction Event

The Permian-Triassic (P-Tr) extinction event, sometimes informally called the Great Dying, was an extinction event that occurred approximately 251 million years ago (mya), forming the boundary between the Permian and Triassic geologic periods. It was the Earth's most severe extinction event, with about 96 percent of all marine species [1] and 70 percent of terrestrial vertebrate species becoming extinct.

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http://en.wikipedia.org/wiki/Permian-Triassic_Extinction_Event