An extinction event (also extinction-level event, ELE) occurs when a large number of species die out in a relatively short period of time.
Since life began on Earth, a number of major mass extinctions have greatly exceeded the background extinction rate present at other times.
Though there were undoubtedly mass extinctions in the Archean and Proterozoic, it is only during the Phanerozoic Eon that the biological invention of bones and shells has provided a sufficient fossil record from which to make a systematic study of extinction patterns.
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