Mass extinctions have been fairly frequent events in the oceans during the course of Phanerozoic time. As many as 15 such events have been recognized in the marine fossil record. Global taxonomic and regional biostratigraphic data show that these mass extinctions have been variable both in severity and in taxonomic groups and geographic areas affected. The Late Permian mass extinction was by far the most severe, affecting nearly all animal groups in most parts of the world. Four other mass extinctions were of intermediate magnitude: the Ashgillian event at the end of the Ordovician, the Frasnian event in the Late...