The earth's worst mass extinction was:

Late Devonian

Ordovician-Silurian

Permian-Triassic

End Triassic

Cretaceous-Tertiary

The Permian-Triassic (P-Tr) extinction event, colloquially known as the Great Dying, the End ... Jump up ^ St. Fleur, Nicholas (16 February 2017). "After Earth's Worst Mass Extinction, Life Rebounded Rapidly, Fossils Suggest". New York Times ...