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, the Latest Permian extinction event, the End-Permian extinction event, and colloquially as the Great Dying, it forms the boundary between the Permian and Triassic geologic periods, and with them the Paleozoic and Mesozoic eras. From Wikipedia
