STRATIGRAPHY AND MICROGASTROPOD PALEOECOLOGY BEFORE THE END-PERMIAN MASS EXTINCTION, PINE FOREST RANGE, HUMBOLDT COUNTY, NEVADA

The largest mass extinction in Earth's history occurred at the end of the Permian period (approximately 250 million years ago). As general interest has increased over the recent years, there have been many proposed hypotheses and mechanisms surrounding this major event, but most remain speculative as there are very few locations in North America where a complete (or nearly complete) Permian-Triassic boundary is exposed for study. The Pine Forest Range in Humboldt County, Nevada has strata of Late Permian and Early Triassic in direct contact with one another, and affords the opportunity to study the environments and paleoecologic trends before the event. The field site was visited in September, 2008 and fossil-bearing rocks from the Permian portions of the section were returned to the laboratory to supplement existing collections. A measured stratigraphic section for the Permian portion of the Pine Forest Range that encloses this fauna is currently being examined for geochemical and geophysical data to define the environments more precisely. Preliminary results include: negative magnetic susceptibility (-0.48 to –1.47 10-7 SI Units), and gamma ray spectroscopy logs (0.5 to 0.7 ppm Potassium; 0.1 to 1.1 ppm Uranium; and 1.3 to 3.4 ppm Thorium), both of which are shown to vary with sea level changes. The highest beds with fossils contain the gastropod, Vesperispira humboldtensis, as the numerical dominant in a fauna of microgastropods (length < 1cm), plus assorted bryozoans, bivalves and brachiopods. All other Permian beds are dominated by the microgastropod, Ananias sp. Size relations for Ananias sp. are consistent with the deep water extension of the biogeographic “island rule”, whereby gastropod genera with larger-bodied shallow-water species have significantly smaller species in deeper water.