A PALEOENVIRONMENTAL STUDY OF FAUNAL RESPONSE WITHIN THE LATE CRETACEOUS HOLZ SHALE, SANTA ANA MOUNTAINS, CALIFORNIA

Faunal changes in response to paleoenvironment perturbation is poorly understood within the Late Cretaceous Holz Shale . We examine faunal patterns across a deepening marine transect to document how organisms respond to environmental change. The Holz Shale Member of the Ladd Formation located in the Santa Ana Mountains of Irvine, CA is exposed along a large monocline that preserves the ancient active continental margin. Exposed continuous beds yield well-preserved and abundant benthic marine invertebrates. Previous paleoecological research conducted by Sundberg (1980) describes a temporal facies relationship that grades from shallow tidal to a proximal continental slope environment. We partook in a collaborative class project in which fossil specimens were collected from 3 bulk rock samples, each representing successively deepening environments of deposition. Specimens were identified to the genus level by comparison using the Sundberg Collection at California State University, Fullerton. The Paleobiology Database (http://paleodb.org) was used to determine life modes and ecologies for the specific genera. First, we calculated Simpson’s Diversity Index and Pielou’s Evenness Index to compare biodiversity and genera distribution between the areas. Second, we calculated Sørenson’s Coefficient of Community Similarity to test faunal pattern similarity between areas. Finally, we calculated Chi-square to test if abundance patterns of Astarte and Syncyclonema in shallow water differed from abundances in deep water. Simpson’s Diversity Index indicates that genera diversity decreases with water depth, while evenness peaks slightly in the environment of intermediate depth. According to Sørenson’s Coefficient of Community Similarity, approximately 50% of the total genera are present in each environment producing intermediate similarity. Lastly, the Chi-square test rejects the null hypothesis that the genera occurring in each environment is similar based on the significant difference in abundance between Astarte and Syncyclonema.