Changes in sample size may confound interpretation of faunal change in the fossil record. The record of early Eocene mammals from the Wasatchian Land-Mammal Age in the Clark’s Fork Basin, Wyoming, shows a high correlation between the square root of sample size and species richness. This correlation suggests that Schankler’s (1980) Biohorizon A, a faunal turnover composed mainly of disappearances, is largely an artifact of sampling fluctuation. Within the interval of Biohorizon A, sample size drops from record high to record low values.

Monte Carlo simulation of the drop in sample size across Biohorizon A demonstrates the role of sampling...