Stability of Regional Brachiopod Diversity Structure across the Mississippian/Pennsylvanian Boundary

Field collections from Arkansas and Oklahoma of paleotropical brachiopod assemblages across the Mississippian/Pennsylvanian boundary reveal a taxonomic turnover event in which the overall diversity structure is conserved. Diversity structure is conserved even though 63% of the latest Mississippian genera go regionally extinct and 50% of the earliest Pennsylvanian genera are either immigrants or newly originated. Taxonomic richness and evenness (Simpson's D) are indistinguishable between the latest Mississippian and earliest Pennsylvanian stages. Additive diversity partitioning shows that the within-collection, between-collections (i.e., within bed), and between-bed diversity components do not change across the Mississippian/Pennsylvanian boundary for richness or evenness. Rank-abundance distributions of genera are the same for both stages. However, for the taxa present in both stages, there is no correlation between their Mississippian abundance and their abundance in the Pennsylvanian. An estimation of spatio-temporal heterogeneity of taxonomic composition (Morisita-Horn index) within each stage reveals that the earliest Pennsylvanian was significantly more homogeneous. These results suggest that middle Carboniferous brachiopod assemblages from tropical shallow-water carbonate platform settings were organized by some factor that was independent of the specific taxa present. Furthermore, the increased homogeneity in taxonomic composition in the earliest Pennsylvanian did not affect the overall diversity structure. Strong interspecific interactions among taxa do not appear to be important in determining the taxonomic compositions and abundances of brachiopod assemblages at the temporal scale of this study.