TAXONOMY, TAPHONOMY AND STABLE ISOTOPE RECORDS OF FRESHWATER INVERTEBRATES FROM THE FLORISSANT FM, COLORADO: TERRESTRIAL RECONSTRUCTION AT THE EOCENE-OLIGOCENE BOUNDARY

Analyses were completed on freshwater invertebrates from the Florissant Formation, 34.07 ± 0.10 Ma, from the Florissant Fossil Beds National Monument. The Florissant Fm. represents a lacustrine and fluvial depositional environment with associated volcaniclastic shales, mudstones, pumice and ash conglomerates. These sediments contain a highly diverse, excellent and uniquely preserved flora and fauna. Taxonomic revisions, taphonomy and carbon and oxygen isotopes were used to reconstruct the past ecology of Lake Florissant. The freshwater Molluscan taxa were revaluated, bringing terminology and identification in line with modern genetic and biologic standards. Taphonomic comparisons of spatially and stratigraphically disparate sites were preformed to confirm taxonomic identifications and to elucidate the quality and diversity of preservation within the lake. Stable isotope analyses were additionally performed on molluscan biogenic carbonates and fibrous calcite to further understand signatures of primary and diagenetic water sources and their potential influences on preservation of calcite within the formation. The taxonomic revision of the molluscan fauna identified two gastropod genera, Stagnicola sp. (Lymnaeidae) and Gyraulus sp. (Planorbidae); and three bivalve genera, Sphaerium sp., Musculium sp. and an unknown genus (Sphaeriidae). Two phenotypes are recognized for Gyraulus sp., with a full spectrum of morphological variability observed. Taphonomic analyses found no differences in population dynamics or preservation quality for Gyraulus sp. across spatially disparate sites in the middle shale unit, though all specimens did show diagenetic alteration. Overall preservation within the overlying caprock conglomerate unit shows unaltered material. δ¹³C and δ¹⁸O data of biogenic carbonates, (i.e. specimens of the Sphaeriidae and Lymnaeidae families), and fibrous calcite show two different signatures: 1) the potential primary water source, and 2) diagenetic pore fluids. Isotope data from the molluscan fauna alone suggests either seasonal variability and/or taxa specific fractionation. The Florissant lake is interpreted as supporting a spatially diverse molluscan ecology, with preservation of original material observed in only one stratigraphic unit.