What was the Permian-Triassic period?

There is no Permian-Triassic period. The Permian and Triassic were two-separate geolgic period. The Permian-Triassic extinction event marks the end of the Permian period and the beginning of the Triassic period. This event was the most devastating mass extinction in Earth's history during which 90% of all life died out. It also marks the end of the Paleozoic era and the beginning of the Mesozoic.