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Extinction is a common phenomenon as shown by the fact that almost all fossil taxa are extinct. But when diverse organism assemblages disappear at, or near, a single geological horizon, it is a temptation to postulate a catastrophic event. Superficial appearances of mass extinction, however, may be misleading. An indication of decline in diversity may not be sufficient to establish the reality of any revolutionary change.

There are many obvious and some obscure sources of error involved in sampling the fossil record. Precise time-correlations are usually lacking and taxon ranges, as recorded, are systematically misleading. The standard method of reporting fossil ranges artificially concentrates last occurrences at stratigraphic boundaries. A similar effect may also result from unrecognized sedimentary hiatuses (paraconformities), which may simulate mass extinction events.

Many biological revolutions are indeed real, as shown by clues of environmental perturbations on a world scale. Generally, however, mass extinctions were spread over millions of years and can be considered catastrophic only as a final disappearance in an accelerating downward trend in diversity.

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