Abstract
A controversial explanation for an apparent 26-million-year periodicity in extinctions over the last 250 million years is that the purported merely reflects extinctions occurring at random. This explanation would be corroborated if it predicted that extinction maxima should occur on the average every 26 m.y. However, if extinctions are occurring at random, then maxima should occur on the average every 19 m.y. rather than every 26 m.y., so the argument loses its force. While a random model does not fit the observed data well, quasi-periodicity need not arise from extraterrestrial causes. An earth-based alternative is proposed in which the number of extinctions during a given time interval comprises a moving average sum of cohort components, one for the interval and for each previous time intervals (up to a threshold N). The magnitude of each such component simply reflects the number of species that originated in the time interval plus the time elapsed since the interval; the former is viewed as a random input and cohorts are considered to follow an exponential pattern of species loss by terminal extinction.
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