Olszewski, Thomas D.¹, Douglas H. Erwin² 
(1) Texas A&M University, College Station, TX 
(2) Smithsonian Institution, Washington, DC

ABSTRACT: Response of Brachiopod Communities (Permian, Glass Mountains, Texas) to Third-Order Sea Level Changes

The Glass Mountains of West Texas contain exceptionally well-preserved silicified brachiopod faunas collected by G.A.Cooper and R.Grant over several decades of fieldwork. In addition, four third-order depositional sequences, spanning ~10 Myr of the Leonardian and lower Guadalupian, have been correlated by previous workers to the well-documented stratigraphic framework of the Guadalupe Mountains. The brachiopods from these sequences are particularly amenable to quantitative paleocommunity analyses for several reasons. First, collections are large, numerous, and diverse (855,047 specimens; 512 species; 142 genera; 191 localities), providing a robust statistical sample. Second, the descriptions and identifications of this material were done at a high level of taxonomic consistency. Third, extraction of the silicified fossils by acid dissolution produced bulk samples appropriate for analysis of species abundances. Ordination of brachiopod data indicates that each third-order sequence has its own distinctive suite of species even though each sequence includes a similar range of carbonate ramp habitats. This suggests that the composition of brachiopod communities changed significantly in response to drops in sea-level changes (recorded stratigraphically as sequence boundaries). In contrast, analysis of the relative species abundances from each sequence indicates a decrease in the proportion of rare taxa in each sequence through the study interval even the overall species richness in individual collections remains high. Loss of diversity due to environmental degradation in modern ecosystems is often due to the disappearance of species with small populations, suggesting an analogous interpretation of the Glass Mountains data through the duration of the study interval. Overall, these findings suggest that the sea level fluctuations governing the deposition of the third-order sequences influenced turnover in species composition, but that other factors determined the structure and composition of each recurrent ecological landscape.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.