Mass Extinctions

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.

percent-genus-extinction

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.