The Permian–Triassic extinction occurred about 252 million years ago. It was the most severe mass extinction known in Earth’s history and marks the boundary between the Permian and Triassic periods.
Often called the “Great Dying,” the event eliminated roughly 90 percent of marine species and about 70 percent of terrestrial vertebrate species, although exact estimates vary. Ecosystems on land and in the oceans took millions of years to recover, making this extinction far more extensive than the later event that ended the non-avian dinosaurs.
The leading explanation involves enormous volcanic eruptions in the Siberian Traps. These eruptions released carbon dioxide, sulfur compounds, and other gases, producing rapid climate warming, ocean acidification, oxygen loss in seawater, and possibly toxic conditions. The effects likely reinforced one another rather than resulting from a single simple cause.
The extinction ended the Permian Period and began the Triassic, the first period of the Mesozoic Era. The 66-million-year figure belongs to the Cretaceous–Paleogene extinction, while 300 and 200 million years ago do not match this boundary event.