Sampling and faunal turnover in early Eocene mammals

Abstract

Faunal turnovers in the fossil record are episodes of synchronous appearance and disapperance of species from a community, often resulting in net change in species richness. We studied the biostratigraphic record of faunal turnover involving early Wasatchian (early Eocene) mammals from the Clark's Fork Basin, Wyoming, U.S.A. Two faunal turnovers occur in this record — one at the base of the Wasatchian, comprised mainly of apperances of taxonomically and ecologically distinctive species, and a later one, Biohorizon A of Schankler (1980), comprised mainly of disappearances, especially of carnivorous species. This study focuses on Biohorizon A. In the record of the Clark's Fork Basin, Biohorizon A may be an artifact of sampling. Sample size and species richness are highly correlated ( r = 0.95) throughout this record. Moreover, sample size and species richness fluctuate markedly between successive stratigraphic intervals; peaks of apperances coincide with large sample sizes and peaks of disappearances with low sample sizes. The peaks and valleys in fossil productivity over time mask the real timing of apperances and disapperances of species. Changes in fossil productivity in the stratigraphic section may result from changes in exposure area, taphonomic factors, or ecological factors. Evaluation of the effects of sampling is a necessary prerequisite for investigating the chronological and ecological significance of faunal turnovers.