The Permian–Triassic extinction event was an extinction event that occurred approximately 251.9 million years ago. Also known as the P–T extinction event, the Late Permian extinction event,[3] the Latest Permian extinction event,[4] the End-Permian extinction event,[5][6] and colloquially as the Great Dying,[7][8] it forms the boundary between the Permian and Triassic geologic periods, and with them the Paleozoic and Mesozoic eras.[9] It is Earth's most severe known extinction event,[10][11] with the extinction of 57% of biological families, 83% of genera, 81% of marine species[12][13][14] and 70% of terrestrial vertebrate species.[15] It is also the greatest known mass extinction of insects.[16] It is the greatest of the "Big Five" mass extinctions of the Phanerozoic.[17] There is evidence for one to three distinct pulses, or phases, of extinction.[15][18]