Olszewski, T. D.,Erwin, D. H.

The silicified brachiopod faunas collected by G. Arthur Cooper and Richard E. Grant over decades of field work in the Permian Basin of western Texas provide a remarkable resource for studying change in the structure of fossilized ecological communities. Ordination analyses using 511 faunal lists that include 967 species (190 genera) from the Kungurian through the Capitanian Stages of the Guadalupe and Glass Mountains (similar to 15 myr) reveal four distinct, major stratigraphic clusters, each divisible into two to three subclusters. For the most part, these clusters coincide with third-order depositional sequences bounded by unconformities. Except for the youngest (Capitanian Stage), all the stratigraphic clusters show internal ordination patterns consistent with biogeographic and environmental (i.e., biotic gradients) control on the distribution of taxa. Despite this evidence for environmental sorting of taxa, Mantel tests indicate that patterns of generic co-occurrence break down from cluster to cluster despite sharing many of the same species and genera. This pattern cannot easily be explained by preservational or stratigraphic biases, suggesting that the processes that govern ecological communities may not lead to unique, strictly determined associations despite the presence of broadly similar species under similar environmental conditions. In contrast to the rest of the study interval, Capitanian clusters are less well structured (i.e., they do not show strong biotic gradients) than assemblages from any of the other clusters and do not show significant recurrence of generic associations. This may reflect evolution of carbonate margins in the basin from ramps to shelves with steep slopes over the duration of the study interval, a shift that may have changed the spatial distribution of (and covariation among) the environmental parameters that controlled brachiopod distributions.