THE COMPLEXITY OF SPECIES ASSOCIATIONS IN PERMIAN BRACHIOPOD COMMUNITIES FROM WEST TEXAS

The silicified faunas of the Permian Basin of West Texas constitute an extraordinary record of late Paleozoic marine diversity. We focus here on the Early and Middle Permian fossil brachiopod communities of the Guadalupe and Glass Mountains (~285-260 Ma). All analyses were performed on 514 faunal lists (967 species from 190 genera) originally collected and documented by G. Arthur Cooper and Richard E. Grant over a period of almost 40 years. Although the number of species they described has been suggested to reflect taxonomic over-splitting, comparison with modern species occurrence distributions does not indicate an excess of rare species but rather suggests that the high species richness of this dataset reflects the size of the collections, not Cooper and Grant's taxonomic procedure. Exploration of the data using ordination reveals four distinct clusters of collections corresponding to four stratigraphic intervals separated by significant amounts of species turnover (lower Leonardian, upper Leonardian-Roadian, Wordian, and Capitanian). Only two of these clusters include data from both the Guadalupe and Glass Mountains (lower Leonardian and Wordian), but the fact that grouping in these cases is stratigraphic rather than geographic indicates that the cause of the apparent faunal changes was basin wide. Continuous gradients of species associations within each cluster are difficult to interpret due to lack of detailed facies and stratigraphic information. Stratigraphic patterns within major clusters could reflect different habitats at different times or, alternatively, the evolution of lineages within stable habitats. Analogously, geographic patterns within major clusters could reflect different habitats in different regions or evolutionary divergence due to geographic isolation of related populations in the same habitats. An additional complication that is yet to be resolved is the degree to which preservation can destroy or create apparent faunal gradients due to selective silicification of brachiopods with different shell microstructure.