The “Big Five”
Five mass extinction events stand out as being more important than the other “minor mass extinctions”. They record times when major environmental change occurred world-wide. Four of the “Big Five” extinctions were at least partly the result of climate change in the form of global warming (end-Permian; end-Triassic) or cooling (end-Ordovician; Late Devonian). The end-Cretaceous event seems to be unique because the environmental effects of the collision of a large (perhaps 6 miles wide) asteroid with the Earth played a role in the extinction.
It is likely that a combination of environmental effects caused the largest mass extinctions. For example, although there is strong evidence for an asteroid impact at the end of the Cretaceous Period, large-scale volcanic activity in India may have contributed by introducing greenhouse gases into the atmosphere, warming the long-term climate. Many mass extinctions occur at times of falling sea level, so that habitat loss is another environmental problem faced by marine animals.
The chart above shows the percentage of groups of species (genera) of marine animals that disappear at each of the “Big Five” extinction events. [Delve deeper: Why do paleontologists count groups like genera instead of species?] Losses of genera are in the range about 40% to nearly 60%. A spike in extinction rates was solely responsible for the end-Ordovician, end-Permian and end-Cretaceous events, but a drop in rate of evolutionary origin of new species also seems to have contributed to the late Devonian and end-Triassic events. The “Big Five” extinctions happened quickly in geological terms, probably over hundreds of thousands of years. If the asteroid impact was the most significant cause of the end-Cretaceous event, the extinctions could have been much faster, perhaps over centuries or even decades.