Mammalian generic diversity and turnover in the late Paleocene and early Eocene of the Bighorn and Crazy Mountains Basins, Wyoming and Montana (USA)
Abstract Patterns of mammalian generic turnover, richness, and faunal composition were investigated for faunas from 17 biostratigraphic zones in middle Paleocene through early Eocene deposits of the Bighorn and Clarks Fork basins of northern Wyoming and the Crazy Mountains Basin of south-central Montana. Significance of turnover was evaluated (1) by comparison of observed turnover to expected turnover (calculated from the multiple regression of turnover on zone duration and generic richness), and (2) by comparison of observed turnover to a bootstrapped turnover distribution. Patterns of turnover and richness also were assessed in light of relative sampling quality of each faunal zone. The analysis identified 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 by a high number of last occurrences in the latest Torrejonian, resulting in a decrease in standing generic richness in the earliest Tiffanian, but no major changes in ordinal composition. During the next interval of significant turnover, the late Tiffanian, higher-than-expected first occurrences resulted in an increase in standing richness and a change in faunal composition, most probably reflecting the immigration of taxa from outside North America. The third, and most dramatic, interval of significant generic turnover took place in the earliest Wasatchian and was distinguished by a high number of first occurrences, but relatively few last occurrences. This led to a marked increase in generic richness, a pattern similar to that for the early Wasatchian of North America as a whole. The major change in faunal composition, as in the late Tiffanian, was largely composed of immigrants from other continents. The pattern of faunal change during the early Wasatchian of the Bighorn Basin, along with evidence for global warming at the Paleocene-Eocene boundary, supports previous interpretations associating this episode in mammalian evolution with the opening of high latitude intercontinental 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 American pattern, and may be, in part, an artifact of still inadequate sampling for the latest part of the stratigraphic sequence in the Bighorn Basin.