MAMMALIAN GENERIC DIVERSITY AND TURNOVER IN THE LATE PALEOCENE AND EARLY EOCENE OF THE BIGHORN AND CRAZY MOUNTAINS BASINS, WYOMING AND MONTANA (USA)

Patterns of mammalian generic turnover, richness, and faunal compositi on were investigated for faunas from 17 biostratigraphic zones in midd le Paleocene through early Eocene deposits of the Bighorn and Clarks F ork basins of northern Wyoming and the Crazy Mountains Basin of south- central Montana. Significance of turnover was evaluated (1) by compari son of observed turnover to expected turnover (calculated from the mul tiple regression of turnover on zone duration and generic richness), a nd (2) by comparison of observed turnover to a bootstrapped turnover d istribution. Patterns of turnover and richness also were assessed in l ight of relative sampling quality of each faunal zone. The analysis id entified four intervals of significant faunal change: the Torrejonian- Tiffanian transition, the late Tiffanian, the earliest Wasatchian, and the middle-to-late Wasatchian. The first interval was characterized b y a high number of last occurrences in the latest Torrejonian, resulti ng in a decrease in standing generic richness in the earliest Tiffania n, but no major changes in ordinal composition. During the next interv al of significant turnover, the late Tiffanian, higher-than-expected f irst occurrences resulted in an increase in standing richness and a ch ange in faunal composition, most probably reflecting the immigration o f taxa from outside North America. The third, and most dramatic, inter val of significant generic turnover took place in the earliest Wasatch ian and was distinguished by a high number of first occurrences, but r elatively few last occurrences. This led to a marked increase in gener ic richness, a pattern similar to that for the early Wasatchian of Nor th America as a whole. The major change in faunal composition, as in t he late Tiffanian, was largely composed of immigrants from other conti nents. The pattern of faunal change during the early Wasatchian of the Bighorn Basin, along with evidence for global warming at the Paleocen e-Eocene boundary, supports previous interpretations associating this episode in mammalian evolution with the opening of high latitude inter continental dispersal routes. During the fourth interval of interest, the middle to late Wasatchian, the Bighorn Basin fossil record shows a drop in generic richness. This differs from the overall North America n pattern, and may be, in part, an artifact of still inadequate sampli ng for the latest part of the stratigraphic sequence in the Bighorn Ba sin.