DISCONTINUITY BUT NO STABILITY IN PERMIAN BRACHIOPOD COMMUNITIES OF WEST TEXAS

The stability of taxonomic associations on geologic timescales is a question with implications for the nature of ecosystem structure and the evolution of lineages in the context of ecological communities. Detecting stability requires numerous samples from a stratigraphic and environmental framework appropriate for the temporal and spatial scales of resolution in question, which has not always been available in previous studies. The Permian brachiopod collections of Cooper and Grant in the National Museum offer an outstanding opportunity to analyze the nature of change in benthic, marine communities in a single basin over a period of 15-20 Ma thanks to their taxonomic consistency, large size, and an independently derived sequence stratigraphic framework. The analyses presented here are based on 967 species from 190 genera in 515 individual collections from the Lower and Middle Permian of the Guadalupe and Glass Mountains in West Texas. Previously presented ordinations indicate that collections from the entire study interval fall into seven distinct stratigraphic clusters separated by third-order sequence boundaries. Within each cluster, there is a great deal of compositional variability among collections, only some of which can be explained by environmental and geographic gradients. Mantel permutation tests of generic associations indicate that stratigraphically adjacent clusters appear to share a significant degree of community structure, but that generic associations are not maintained over the longer duration of the entire study interval. Overall, the Permian Basin appears to show distinct ecological-evolutionary subunits (the stratigraphic clusters), but collections within these units show a great deal of both explainable and apparently random variability in terms of taxonomic composition. Over the longer term, taxonomic associations within clusters appear to evolve and change rather than remain static. Only large, stratigraphically and taxonomically well-constrained, collections-based data sets like that assembled by Cooper and Grant provide the resolution necessary to document and test hypotheses of ecological community change in the fossil record.