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Fossil assemblages are sensitive indicators of environmental changes preserved in sedimentary rocks. As such, an understanding of the ecological processes that influence the diversity and composition of fossil assemblages can provide insight on the magnitude and character of ancient sea level changes that fashion the stratigraphic architecture of sedimentary successions.

Within Guadalupe Mountains National Park, carbonate deposits of the Middle Permian Bell Canyon Formation contain abundant brachiopods. The carbonate members of this formation (from bottom to top: Hegler, Pinery*, Rader*, McCombs, Lamar*, and Reef Trail*; asterisks indicate members that were sampled as part of this study) are deep-water deposits that accumulated in a basin rimmed by an extensive reef. Two third-order sequence boundaries, indicating episodes of regional-scale environmental disruption related to sea level changes, have been recognized within the Bell Canyon Formation: one between the Pinery and Rader Members and another between the Rader and McCombs Members. However, the correlations of these surfaces between the reef platform and the deep basin have remained ambiguous due to difficulties in identifying them in the massive reef facies. The existing sequence framework in the region and extensive previous documentation of Permian Basin brachiopods provides an opportunity to test the application of modern ecological theories of community response as a tool to interpret the magnitude of ancient environmental changes.

Based on multivariate analysis of materials collected over two six-week seasons of fieldwork in the park (as well as several additional short visits), brachiopod paleocommunities of the Pinery and Rader Members differ from paleocommunities of the Lamar Member. Paleocommunity change in the Lamar Member documents the displacement of established brachiopods with new taxa, including the invasion of two brachiopods from outside the basin. The Reef Trail Member, the uppermost member of the Bell Canyon Formation, shows different taxonomic associations than the underlying members, but this represents a reorganization of previously occurring taxa (no new taxa are introduced). These results indicate that the composition of the regional species pool (i.e., the pool of species from which taxa in local paleocommunities are drawn) remained relatively similar in the Pinery, Rader, and Reef Trail Members, but the Lamar paleocommunities drew on a different pool of species.

In addition to compositional differences, multivariate analyses also indicate that local paleocommunities from the Lamar Member are more similar to one another than in the other members. A diversity partitioning approach was used to assess the distinctiveness of local paleocommunities within each member: if most of the total diversity in a member could be found in a local community, this would suggest that dispersal was relatively easy and was an important influence on the structure of local paleocommunities (i.e., species could easily migrate from one community to another). If local communities were very different and individually only accounted for a small fraction of the total diversity in a member, this would suggest that local communities could competitively exclude potential immigrants. From the Pinery through the Rader to the Lamar, there is a decrease in the distinctiveness of local paleocommunities (i.e., local paleocommunities contain an increasing proportion of the regional species pool in each successive member). In contrast, brachiopod diversity of the Reef Trail Member shows a sharp drop in the diversity of local paleocommunities and a corresponding increase in their compositional distinctiveness. These changes in the partitioning of diversity through time suggests an increase in the effectiveness of dispersal of brachiopods from the Pinery to the Lamar Member followed by a decrease in effectiveness of dispersal in the Reef Trail Member. This pattern emerges despite the fact that the Lamar Member has the highest total diversity of brachiopods compared to the other members (i.e., it would be easier for local communities to be distinctive).

Implications from this study suggest that the compositional and diversity changes of the Lamar Member paleocommunities appear to be facilitated by sea-level changes. The inferred fall in sea level associated with the McCombs sequence boundary appears to have caused large-scale disruption to previous community states that made it possible for new taxa to colonize the slope and for previously present taxa to change their habitats. The ecological changes identified in this study suggest that the sequence boundary associated with the McCombs (or possibly the base of the Lamar) represents a fundamentally different scale of disruption than the other carbonate members of the Bell Canyon. The ecological communities suggest that this event disrupted the species pool and community organization of the entire Permian Basin and should therefore be extensively correlatable. Recognition of this surface using modern ecological metacommunity theory (i.e., the body of theory concerned with the relationships among local communities in a broader regional-scale context) provides a new tool to correlate surfaces from the carbonate platform to those deep in the basin, which have previously been ambiguous but are critical to understanding the geometries of deep-water reservoir and source facies.