Illustration of a Gorgonopsid scavenging on a therapsid carcass during the Permian extinction around 252 million years ago. Known as The Great Dying about 95 percent of marine species and 70 percent of terrestrial species were wiped out, making it the most severe of the four or five mass extinctions which have occurred in Earths history. The cause is thought to be a series of volcanic eruptions that released huge amounts of sulphur dioxide and carbon dioxide into the atmosphere, reducing oxygen levels and acidifying the oceans.