Abstract

The fossil record is necessary to provide an extended temporal scale to studies of community stability and equilibrium and species longevities and origination. We addressed these topics with a large database for Cenozoic benthic foraminifera from six formations (ranging from Eocene to Pliocene) occurring in the Salisbury and Albemarle embayment(s) (SAE) of the North American Atlantic Coastal Plain. We also documented the worldwide fossil occurrences of the 356 species found in these formations to 142 well-dated formations, most of which are in the Atlantic and Gulf Coastal Plains and which comprise a species pool from which species are drawn during each transgression into the SAE. A ledger was constructed with the first entry consisting of the number of immigrants to each formation, and the last the number of emigrants. Consideration of the ledger indicates a dynamic system. Faunal stability within ecological units, rapid and continual change, and balance and imbalance of immigrants versus emigrants are all observed. During transgressions, immigrants enter the SAE from the source species pool and, on the average, about half of the available species inhabit the SAE during a transgression. During regressions, emigrants disperse into various parts of the wide geographic area of the pool and about a quarter never return to the SAE. The distribution of each species' occurrence is unique in space and time. Emigrant species do not migrate to another location as a unit nor do they immigrate as a unit. Consequently, no recourse to biotic interactions or hierarchical structure is required to explain the observations presented here. Rather, environmental regime plays an important role in affecting community composition.

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