Ordovician-Silurian extinction, global extinction event occurring during the Hirnantian Age (445.6 million to 443.7 million years ago) of the Ordovician Period and the subsequent Rhuddanian Age (443.7 million to 439 million years ago) of the Silurian Period that eliminated an estimated 85 percent of all Ordovician species. This extinction interval ranks second in severity to the one that occurred at the boundary between the Permian and Triassic periods about 251 million years ago in terms of the percentage of marine families affected. The Ordovician-Silurian extinction was almost twice as severe as the K–T extinction event that occurred at the end of the Cretaceous Period, about 65.5 million years ago, which is famous for bringing an end to the dinosaurs.

Extinction events in the ordovician period