For any one species, extinction may seem catastrophic. But over the grand sweep of life on Earth, extinction is business as usual. Extinctions occur continually, generating a "turnover" of the species living on Earth. This normal process is called background extinction. Sometimes, however, extinction rates rise suddenly for a relatively short time an event known as a mass extinction. Mass extinctions kill off many species, but the empty niches left behind may allow other lineages to radiate into new roles, shaping the diversification of life on Earth.
With the data available now, it appears that life on Earth has experienced several mass extinctions. The most devastating, perhaps, was the Permian mass extinction 225 million years ago. About 90% of the species living at the time went extinct, including most of the dominant land vertebrates at the time, the synapsids. Scientists are studying this event to figure out whether it was caused by climate change, volcanic eruptions, sea level change, continental movement, asteroid impacts, or several of these factors.
The K-T extinction
The K-T extinction (Cretaceous-Tertiary extinction, 65 million years ago) is famous for killing off the dinosaurs (except birds, of course!), along with two-thirds of all species on Earth. However, small mammals, turtles, crocodiles, birds, redwood trees and many others survived.
There is very good evidence that a giant asteroid hit Earth at the same time
as the K-T extinction. The smoking gun is the 100-mile wide crater it left
behind off the coast of Mexico, along with disturbed geologic deposits (iridium and shocked
quartz) consistent with an asteroid impact. The impact probably
tidal waves, earthquakes, and clouds of dust so thick that they blotted
out the sun for months. Such a disaster is certainly capable of causing
extinction. However, many lineages were on the wane, experiencing a lot
of extinction, even before the asteroid hit. These pre-impact extinctions
have other causes.