Mass extinction

A mass extinction is an event in Earth history in which an unusually large number of species go extinct in a short period of time. The rough opposite of a mass extinction is an adaptive radiation, a period when an unusually large number of new species arise.

Great Extinctions
Since 1982 we have identified the most deadly mass extinctions during the Phanerozoic Eon, those that killed well over half of all species of life on Earth, as Great Extinctions. There have been five of these, and we may now be in the midst of a sixth:


 * First Great Extinction: 450–440 million years ago, at the Ordovician-Silurian boundary. 60-70% of all species died.
 * Second Great Extinction: 375–360 million years ago, in the late Devonian Period. 70% of all species died.
 * Third Great Extinction: 251 million years ago, at the Permian-Triassic transition. Also known as the "Great Dying," this horrific event killed over 90% of all species.
 * Fourth Great Extinction: 200 million years ago, at the Triassic-Jurassic transition. 70-75% of all species died.
 * Fifth Great Extinction: 65 million years ago, at the end of the Cretaceous. The best-known mass extinction event because most dinosaur species died (birds were the only exceptions). 75% of all species died.
 * Sixth Great Extinction: The likely result of current trends in extinctions resulting from human activities. It's not clear how many species have died so far, but thousands are going extinct every year.