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Summary: The disappearance of nonavian dinosaurs is probably the most
notorious extinction event of all time, yet it is only a small part
of a greater class of extinctions known as "mass extinctions."
Mass extinctions are global events characterized by unusually
high rates of extinction. The magnitude of these rates is usu-
ally unspecified but it is generally significantly higher than
the rate of so-called "background extinctions;" that is, extinc-
tions that occur constantly through geologic time (Raup and
Sepkoski 1982). Mass extinctions are also characterized by geo-
logically short timescales and by a significant diminution in
the number of surviving taxa, as well as in their diversity. Al-
though background extinctions may account for more than
95% of all extinctions through geologic history, it appears that
mass extinctions reset the evolutionary clock, as it were, allow-
ing one major taxonomic group to replace another in a given
environment.
Five episodes of mass extinctions are generally regarded as
the most significant in Earth history. These are (in order of
decreasing magnitude) the Permo-Triassic extinction (245 mya),
the Late Ordovician extinction (~439 mya), the Late Devonian
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