http://www.bagheera.com/inthewild/spot_massextinctions.htm
The five mass extinction episodes occurred because of major changes in the prevailing ecological conditions brought about by climate change, cataclysmic volcanic eruptions, or collisions with giant meteors.
Millions of Years Ago | Name of Extinction Episode | Percent of Families That Died Out | Note |
440Late Ordovician20 - 50 %Tropical areas are hit hardest. No terrestrial vertebrates have evolved yet.360Late Devonian 20 - 30 %Many fish and marine invertebrates die out.250End of Permian 50 %Greatest mass extinction ever.213Late Triassic20 - 35 %Dinosaurs and mammals had just recently evolved; both live through this extinction.66Late Cretaceous15 %Dinosaurs become extinct, possibly caused by a metorite colliding with the Earth. Mammals and fishes begin to proliferate. 65 million of years after this mass extinction, early humans evolve.