COMPARING INCUMBENT AND INVADER BRACHIOPOD TAXA WITH ABUNDANCE AND BIOVOLUME ESTIMATES WITHIN THE BELL CANYON FORMATION (MIDDLE PERMIAN) OF THE DELAWARE BASIN (GUADALUPE MOUNTAINS, WEST TEXAS)

Brachiopods of the Delaware Basin (Guadalupe Mountains, West Texas) record a shift in the dominant taxa within metacommunities (group of local communities connected through dispersal) of the Middle Permian Bell Canyon Formation. Based on presence/absence data, a compositional change within the upper part of the formation documents the displacement of established brachiopods by previously rare and immigrant genera. This study measures the abundance and body volume of brachiopods to investigate the diversity change of the brachiopod fauna within the Pinery, Rader, Lamar, and Reef Trail Members. Abundance and body volume carry information on community-level properties, such as resource allocation, and therefore provide useful information on community assembly and diversity. For use in this study, 53 brachiopod genera and 6,038 individuals were identified. The collections for each member are pooled together to represent a metacommunity. Body volume (or biovolume) is estimated from the ATD model of Novack-Gottshall (2008). The generic rank-abundance plots for each member are similarly shaped and cannot be statistically distinguished based on a Kolmogorov-Smirnov two-sample test. Although these metacommunities have a consistent abundance structure through time, there is a difference in the dominant taxa or taxon in each metacommunity. The brachiopod Hustedia is the dominant brachiopod in the Pinery and Rader Members, whereas Martinia and Astegosia are the dominant genera in the Lamar Member. In the Reef Trail Member, two different genera, Derbyia and Crurithyris, replace the Lamar Member dominants. Mean biovolume of Hustedia varies through time, but does not correlate with its abundance. There is a significant drop in mean biovolume of Hustedia (incumbent) from the Rader Member to the Lamar Member when Astegosia (invader) appeared in the metacommunity with high biovolume and abundance. Biovolumes of these two brachiopods are statistically different in the Lamar Member. Mean biovolume of Astegosia increased in the Reef Trail, but it cannot be statistically distinguish from Hustedia biovolume. These results suggest that the invading taxa with high biovolume colonized the metacommunity and changed interactions among brachiopods to become dominant, but these interactions may have weaken over time.