Mollusc-Based Interpretations of Lacustrine Paleoenvironments of the Sheep Pass Formation (Latest Cretaceous to Eocene) of East Central Nevada

Abstract

The slow supra-species level evolution of nonmarine molluscs permits interpretation of paleoenvironments from the environmental constraints of taxonomically similar modern nonmarine molluscs. Therefore, nonmarine fossil molluscs provide a valuable paleoecological tool for understanding ancient depositional environments and their spatial and temporal distributions. This enhanced understanding of paleoenvironments combined with lithostratigraphy allows interpretation of the climatic and tectonic conditions that affected ancient lacustrine systems. Empirical and quantitative cluster analyses have defined recurring groups of molluscs (associations) from mollusc assemblages in the lacustrine Sheep Pass Formation complex. Molluscan associations were verified through taphonomic analyses. The paleoecology of molluscan associations is interpreted ly taxonomic uniformitarianism applied at both autecological and synecological levels. The Valvata-Hydrobia-Sphaeriidae (VHS) association has low diversity with only one species of each of these taxa per sample. These taxa are all gill-breathing and respire subaqueously. Modern analogs of the VHS association inhabit shallow, lowenergy lacustrine environments lacking emergent vegetation. The Lymnaeidae-Biomphalaria (LB) association has high species diversity and commonly occurs with terrestrial gastropods in an ecologically mixed assemblage. The taxa composing the LB association are lung-bearing and generally respire at the airwater interface. Modern analogs of the LB association inhabit shallow, ephemeral lacustrine environments with abundant emergent vegetation. A fauna analogous to the LB association occurs with terrestrial gastropods in marly bog lakes of Maine and form ecologically mixed assemblages similar to the assemblage of the LB association. The VHS association occurs stratigraphically below the LB association. The change in molluscan fauna indicates changing environments within the Sheep Pass lake basin, from a single larger lake with no emergent vegetation to numerous small ponds with abundant emergent vegetation within a wetland terrain. INTRODUCTION