Of the five mass extinction events commonly recognized in Earth history, in which exceptionally large numbers of species were lost within short intervals of time, four coincide with period boundaries. The most dramatic, which almost destroyed life altogether, occurred at the end of the Permian Period, currently dated at 252 million years ago, while the most famous, the mass extinction of the non-avian dinosaurs and marine reptiles, occurred at the Cretaceous–Palaeogene boundary (formerly known as the K-T boundary), 66 million years ago.
