Preservational, paleoecological and evolutionary patterns in the Paleogene of Wyoming-Montana and the Neogene of Pakistan

Abstract

Abstract Contributions to this special issue have compared the tectonic settings, fluvial systems, paleoclimates, paleofloras and faunas, vertebrate taphonomy, and mammalian paleoecology and evolution from two long, continental records of Cenozoic ecosystems. In this concluding paper, we summarize highlights of earlier papers to provide an overview of similarities and differences between these Paleogene and Neogene records. The relative influences of tectonic, climatic, and fluvial processes on lithofacies, environments of fossil preservation, and the productivity of the fossil record are compared for each sequence. Tectonism was the primary control on sediment accumulation rate and the distribution of major depositional environments in each basin. Fossil productivity, species richness, and the distribution of fossil localities among fluvial environments change across formation boundaries in each sequence. The taxonomic, temporal, and spatial resolution of vertebrate assemblages varies among fluvial environments and differs between the two sequences. As a result, the Paleogene record is better suited for detailed study of evolutionary change within local lineages and the Neogene record for detailed paleocommunity reconstruction. Several aspects of biotic 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 change was accompanied by change in faunal composition and ecological structure, but both pulsed and diachronous biotic change, at a resolution of about 0.5 m.y., also occurred over periods when no significant climatic change was discernible. This pattern best matches the predictions of the Red Queen model. (2) In both records, the rate of mammalian faunal turnover (first and last appearances of taxa) is not highly correlated with standing richness, suggesting that paleocommunities were not a equilibrium richness on these time scales. (3) In the Paleogene record, plant species richness declined while mammalian species richness increased. (4) In each sequence, some episodes of mammalian faunal turnover were in step with changes in size or trophic structure, while other changes in faunal composition entailed no corresponding change in ecological structure. While both records would benefit from improved paleoclimatic, temporal, species-level, and ecomorphological resolution, this initial synthesis suggests that both physical and biotic factors were important influences on faunal composition, the timing and rate of turnover, and ecological structure in these paleocommunities.