CROSS-SCALE TEMPORAL AND SPATIAL DIVERSITY AND STRUCTURE OF MOLLUSCAN FAUNAS FROM THE PLIOCENE ETCHEGOIN GROUP, SAN JOAQUIN BASIN, CENTRAL CALIFORNIA

Presence-absence molluscan faunal data from 484 localities were compiled into 116 ten-meter stratigraphic sample intervals, 14 samples by 4^th-order eustatic cycle, three samples by formation, and the Etchegoin group in toto, to develop spatial and temporal diversity models for the Pliocene San Joaquin Basin (SJB). The fauna consists of 171 taxa with 19% of species accounting for 67% of all occurrences. Endemic species, extinct species without a record outside of the SJB, comprise 29% of the fauna. Shannon diversity (H) and evenness (E) were calculated for the total and endemic faunas from each locality collection and sample interval. Locality collections (α₁ diversity) contribute 62% of diversity within ten-meter sample intervals (α₂) reflecting regional habitat patchiness. Endemic species account for 42% of α₂ diversity indicative of their environmental sensitivity. Diversity between 4^th-order eustatic cycles (β₁) accounts for 80% of total diversity (γ) consistent with the eustatic control of faunal composition and complex species associations coincident with highstands and normal marine conditions and very patchy species distributions corresponding to brackish conditions at lowstands and SJB shoaling during upper Etchegoin group deposition. Diversity due to extinctions (β₂) is associated with eustatic regression at formation boundaries. Regional variations in H and E are interpreted as associated with tidal-flat and bay environments and substrate-controlled patchy habitat distributions consistent with modern intertidal and nearshore mollusc faunas. Partitioning α₂ diversity between endemic and non-endemic faunas reveals habitat segments as shared or available solely to endemic species. During eustatic cycles of transgression and regression, available endemic habitats expand and contract, respectively, at a greater rate than the shared habitat. Invading generalist species quickly fill the shared habitat during transgression and displace endemic species during regression. The implication for modern depleted endemic faunas of shallow-coastal and ocean-marginal environments is that during the current period of global sea level rise displacement into the shared-habitat with consequent extinction likely if adaptation does not keep pace with environmental change.