Degree Granting Department
Peter J. Harries, Ph.D.
Neogene, Chronostratigraphy, Paleoecology, Multivariate statistics, Diversity
This study focuses on reconstructing the dynamics within the Pliocene San Joaquin Basin (SJB) molluscan fauna. This was accomplished by 'binning' the data within a constrained chronostratigraphic framework into: 1) 484 individual stratigraphically-ordered locality collections; 2.) 116 stratigraphically-sequential compiled ten-meter sample intervals; 3.) 15 intervals compiled by 4th-order eustatic cycles; 4.) three formation-level compiled samples; and 5.) the Etchegoin group fauna (informal San Joaquin Basin nomenclature) overall. These datasets were analyzed by inferential, multivariate, and descriptive statistics to examine local and regional environmental controls on faunal composition, community associations and distributions; cross-scale faunal structure; and large-scale environmental controls on immigration, diversity, and extinction. Primary environmental controls on community composition and spatial distribution were substrate type and water paleo-depth.
Consequently, the Pliocene SJB record is one of a temporal succession of complexly distributed habitats and species. Regional habitat patchiness controlled individual locality-level (a1) diversity and contributed 62% of regional sample-level (a2) diversity. Endemic species comprise 30% of the fauna but account for 42% of a2 diversity, indicative of their environmental sensitivity. Partitioning a2 diversity between non-endemic and endemic species reveals habitats segmented as shared or available solely to endemic species. At the level of 4th-order eustatic variations, diversity between temporal samples (b1) accounts for ~80% of total (y) diversity consistent with eustatic control of faunal structure. During eustatic fluctuations, endemic habitats expanded and contracted at rates greater than shared habitats. Invading species quickly filled shared habitat during transgression and displaced endemic species during regression.
Therefore, climatic- and regression-driven hydrologic change and productivity collapse in the Pliocene SJB led to seven extinctions of >40% species. Peak faunal diversity corresponded to periods of highest sea-levels whereas low-diversity faunas characterized low to rising sea levels. Thus, speciation events following extinctions suggest diversification of surviving faunas into habitats newly-created by changed environmental conditions.The broader implication of this study is that during current global sea level rise depleted endemic faunas of shallow-coastal and ocean-marginal environments will be displaced into the shared-habitat with consequent extinction likely if adaptation does not keep pace with environmental change.
Scholar Commons Citation
Bowersox, John Richard, "Community structure, faunal distribution, and environmental forcing of the extinction of marine molluscs in the Pliocene San Joaquin Basin, Central California" (2006). Graduate Theses and Dissertations.