| The aim of this project was to test for a relationship between paleoecological community stability and rates of taxonomic turnover using bivalves and brachiopods of the Pennsylvanian-Permian Midcontinent (Kansas and Nebraska). The first of three main parts focused on building a high-resolution time-environment framework for the upper Wabaunsee, Admire, and lower Council Grove Groups (∼2.5 Myr). Applying the principles of sequence stratigraphy revealed two scales of cyclicity—51 meter-scale cycles and 5 composite sequences. Meter-scale cycles are genetic packages bounded by subaerial unconformities; lowstand systems tracts are preserved as incised valley fills, transgressive systems tracts are dominated by nearshore to offshore carbonate deposits, and highstand systems tracts are dominated by nearshore to offshore siliciclastic deposits. Correlation of these cycles reveals composite sequences that show a rapidly deepening to slowly shallowing succession of meter-scale cycles bounded by angular unconformities. This high-resolution time-environment framework served as the foundation for paleoecological analysis, the second part of this study. Exploratory statistical techniques (cluster analysis and correspondence analysis) revealed that bivalves favored nearshore settings and brachiopods favored offshore settings on the Midcontinent Platform. Monte Carlo and bootstrapping methods indicate that the degree of recurrence of faunal gradients within each of these two paleocommunities is not significantly different. In the third part of the study, taxonomic turnover in the two paleocommunities was studied over a ∼17.5 Myr interval divided into 32 cycles (each ∼500,000 years). No significant difference in the rate of background disappearance could be detected in the two groups, but a significant episode of elevated first appearances occurs in the bivalves during early Missourian time and a significant disappearance event occurs in the brachiopods during early Wolfcampian time. In conclusion, different turnover histories in the two groups do not correspond to different degrees of paleoecological community stability (as measured by recurrence). This indicates that processes governing community level patterns of taxonomic associations are different than those governing long-term faunal change in this case. |
