LATE PALEOZOIC MICROGASTROPOD-DOMINATED FAUNAS OF THE PINE FOREST RANGE, HUMBOLDT COUNTY, NEVADA

An unnamed limestone formation in the Pine Forest Range of Humboldt County, Nevada, contains an abundant, silicified, microgastropod assemblage. The presence of diagnostic macrofossils, including Omphalotrochus and fusulinids constrain the age of the base of the formation to Late Carboniferous through Early Permian. The fauna is found within skeletal packstones and wackestones that are segregated into discrete debris flow horizons. The flows are interpreted to have been deposited in a distal, cool-water carbonate ramp environment. The fauna contains two microgastropod-dominated paleocommunities: the Ananias – Vesperispira paleocommunity and the Microdoma paleocommunity. The taxonomic, paleoecologic and taphonomic composition of these communities is consistent with a deep-water interpretation for the original paleoenvironment. Over twenty distinct species of microgastropods (height less than 1cm) numerically dominate a fauna that also contains bivalves, scaphopods, bryozoans, corals, crinoid columnals brachiopods and annelids. Small sizes for the mollusks may be attributed to reduced growth in local, stressed environments of deposition with some size sorting by debris flow sedimentation. Probabilistic analysis of gastropod genera of the Pine Forest Range paleocommunities reveals paleobiogeographic affinities with other microgastropod-dominated faunas in the accreted terranes of western North America and the Tethyan faunas of Asia.