INFLUENCE OF SEA LEVEL CHANGES ON DIVERISTY OF PERMIAN BRACHIOPOD COMMUNITIES (GLASS MTNS, TEXAS)

A critical reason for understanding global change is its effect on ecosystem properties like diversity and productivity. Ecologists address these issues using small-scale experiments and by monitoring modern communities. The fossil record, however, preserves the results of far larger natural experiments on the effect of environmental changes on fossil assemblages. Leonardian and Guadalupian rocks of the Glass Mountains (west Texas) record sea-level changes at several orders of magnitude and also contain exceptionally well-preserved silicified brachiopod faunas. The collections from this region, made by G.A.Cooper and R.Grant over several decades, are large and diverse (855,047 specimens; 512 species; 142 genera; 191 localities), providing a robust statistical sample. All fossil material analyzed in this study was extracted by acid dissolution of the carbonate rock matrix. The resulting bulk samples allowed description and identification of fossil material at a high level of taxonomic resolution and consistency due to the excellent preservation of silicified specimens and are also appropriate for analysis of species abundances. The succession includes four third-order depositional sequences, each with its own distinctive suite of species but a similar range of carbonate ramp habitats. In addition, the interval records a second-order rise and fall in sea level (recorded by the stacking pattern of the third-order sequences). Application of a neutral model of biodiversity indicates that landscape-scale ecological dynamics changed in concert with second-order sea level change: higher sea level coincided with greater connectivity of local communities in the region as well as higher rates of species replacement by speciation. The observed patterns suggest that ecological communities can accommodate changes in environment, associated with second-order fluctuations in sea level in this case, through adjustments in rates of species production and immigration while remaining diverse in terms of taxonomic and functional composition.