The appearance of a new species or disappearance of an old species may be considered a biostratigraphic event. If simultaneity does not exist, each first and last occurrence can be separated into a sequence of events. If dispersal rates are high compared with evolutionary rates, the sequence should be the same in all sections. The stratigraphic record is then divisible into several increments ("concurrent range zones") equal to the number of events minus one. Most fossils do not satisfy the requirements of occurrence and distribution, but calcareous nannofossils are admirably suited for refining biostratigraphic resolution. For example, Upper Cretaceous strata contain about 82 distinctive species of coccoliths which have their first and last occurrences in Cenomanian-Maes richtian strata, so that as many as 163 increments may be distinguishable in this interval.

Biostratigraphic correlation now may be defined as the probability that a specified assemblage belongs at a certain point in a sequence of events. The correlation depends on three factors, each of which may be expressed in statistical terms: (1) the probability that the events used to define a biostratigraphic increment are in the true order, (2) the probability that the species used to define the superjacent or subjacent increment is absent, and (3) the probability that the critical species determinations are correct.

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