IDENTIFYING THE PRIMARY FACTOR IN PALEOCOMMUNITY STRUCTURE: A CASE-STUDY OF LATERAL VARIATION IN THE IMPERIAL FORMATION OF THE COYOTE MOUNTAINS IN IMPERIAL COUNTY, CALIFORNIA

Within the Imperial Formation in Alverson and Fossil Canyons, in the Coyote Mountains of Southern California, communities dominated by oysters and corals are within 95 meters of each other. This small field site, ~0.32 sq. kilometers, provides an opportunity for a unique study of the primary factor causing the lateral variation in paleocommunity structure. A majority of community-level marine studies have assumed the depth of the water column to be the primary physical force in paleocommunity structures, but only a handful of studies test this hypothesis. Community-level studies also tend to recognize that some degree of lateral variation exists, but few explain the cause behind it. The goal of this study is to capture the lateral variation and determine the primary factor in paleocommunity structures. The role of depth, lithology, organisms’ adult mode of life, and predator- prey relationships were all tested. This study was based on 21 samples that yielded 35 taxa, dominated by bivalves and gastropods, collected in situ from the lower 8.5 meters of the Imperial Formation. Taxa were identified to the genus level using published descriptions; ecological and behavioral information were inferred from modern analogues.

Paleocommunity structure was identified using cluster analysis based on similarities between taxa abundance and sample sites. Relative Sorenson, Sorenson, and correlation distance measurements of abundance and presence/absence data matrices were preformed to determine if clustering patterns were robust with respect to methodology. The resulting clustering patterns are consistent, regardless of the distance measurement and linkage method. Depth and lithology were not the primary factor in community development. Clustering patterns were strongly influenced by lateral variation. The lateral variation of paleocommunity structures resulted from organisms’ adult life mode. Taxa clustered into four different adult modes of life: deep infaunal, shallow infaunal, unattached or weakly attached epifaunal, and cemented epifaunal. Predator-prey relationships explain the positioning of predatory gastropods. Community-level studies need to account for the scale of the project area and consider that lateral variation, not necessarily depth, may be the primary factor in paleocommunity structures.