Diversity Dynamics of Brachiopods in the Delaware Basin (West Texas) Prior to the End-Guadalupian Extinction

Assessing the distribution of and relationships among species in ecological communities can provide insight into the processes that shape biodiversity. This study compares the taxonomic composition of brachiopod communities among members of the Bell Canyon Formation (Capitanian) in the Delaware Basin of West Texas. The purpose of this study is to investigate regional diversity dynamics prior to and leading up to the end-Guadalupian extinction. The Bell Canyon Formation (~5.4 myr) records deposition on a carbonate slope by sediment-gravity flows seaward of the Permian Reef. The members record environmental conditions just before the Delaware Basin changed from open-marine circulation with diverse brachiopod communities to restricted, hypersaline conditions with no brachiopods (i.e., the overlying evaporites of the Castile Formation). The uppermost member (i.e., Reef Trail) is hypothesized to preserve marine environments that may show the initial effects of basinal restriction. Additionally, previous sequence stratigraphic work on these rocks recognizes two third-order sequence boundaries within the Bell Canyon that record environmental perturbations. Eight bulk samples were collected from the Pinery, Rader, Lamar, and Reef Trail Members on the west, central, and east sides of Guadalupe Mountains National Park. Silicified fossils were extracted from limestone using 10% HCl in a tub lined with ~850μ mesh. Whole and fragmented brachiopods were identified to the generic level. To investigate differences in taxonomic composition, brachiopod presence/absence data were analyzed using non-metric multidimensional scaling (NMDS) and cluster analysis. Results from the Pinery and Rader indicate that brachiopod communities have similar taxonomic compositions although they are separated by a third-order unconformity, suggesting either uniform conditions through time or that differences in conditions do not affect taxonomic compositions. In contrast, brachiopod communities appear to change in the overlying Lamar and Reef Trail Members, suggesting a fundamentally different ecological response relative to previous perturbations driven by sea level change.