PRESERVATIONAL, PALEOECOLOGICAL AND EVOLUTIONARY PATTERNS IN THE PALEOGENE OF WYOMING-MONTANA AND THE NEOGENE OF PAKISTAN

Contributions to this special issue have compared the tectonic setting s, fluvial systems, paleoclimates, paleofloras and faunas, vertebrate taphonomy, and mammalian paleoecology and evolution from two long, con tinental records of Cenozoic ecosystems. In this concluding paper, we summarize highlights of earlier papers to provide an overview of simil arities and differences between these Paleogene and Neogene records. T he relative influences of tectonic, climatic, and fluvial processes on lithofacies, environments of fossil preservation, and the productivit y of the fossil record are compared for each sequence. Tectonism was t he primary control an sediment accumulation rate and the distribution of major depositional environments in each basin. Fossil productivity, species richness, and the distribution of fossil localities among flu vial environments change across formation boundaries in each sequence. The taxonomic, temporal, and spatial resolution of vertebrate assembl ages varies among fluvial environments and differs between the two seq uences. As a result, the Paleogene record is better suited for detaile d study of evolutionary change within local lineages and the Neogene r ecord for detailed paleocommunity reconstruction. Several aspects of b iotic change are considered. (1) The timing and magnitude of mammalian faunal turnover in relation to climatic change are evaluated in terms of three models of evolutionary change within ecosystems: Van Valen's ''Red Queen model,'' Stenseth and Maynard Smith's ''stationary model, '' and Vrba's ''turnover-pulse model.'' In both records, climatic chan ge was accompanied by change in faunal composition and ecological stru cture, but both pulsed and diachronous biotic change, at a resolution of about 0.5 m.y., also occurred over periods when no significant clim atic change was discernible. This pattern best matches the predictions of the Red Queen model. (2) In both records, the rate of mammalian fa unal turnover (first and last appearances of taxa) is not highly corre lated with standing richness, suggesting that paleocommunities were no t at equilibrium richness on these time scales. (3) In the Paleogene r ecord, plant species richness declined while mammalian species richnes s increased. (4) In each sequence, some episodes of mammalian faunal t urnover were in step with changes in size or trophic structure, while other changes in faunal composition entailed no corresponding change i n ecological structure. While both records would benefit from improved paleoclimatic, temporal, species-level, and ecomorphological resoluti on, this initial synthesis suggests that both physical and biotic fact ors were important influences on faunal composition, the timing and ra te of turnover, and ecological structure in these paleocommunities.