Frequency distributions of biostratigraphic gaps are generated for eight fossil species. The distributions help estimate confidence intervals for biozonation boundaries by providing information on the size of the gap one may reasonably expect between the lowest and highest visible fossil occurrences, seen in cross section, and the actual lowest and highest individuals, unseen within the rock, which occur beyond them. The gaps often follow a random (Poisson or near-Poisson) distribution, except for a few gaps that occur outside the distribution. These latter seem to indicate periods of abnormal increase in the rate of sedimentation and/or decrease in faunal abundance.

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