An Intensive investigation of the Fossil Hill Member (northwestern Nevada, United States) leads to the recognition of new ammonoid faunas that bracket the Middle/Late Anisian boundary. These faunas are of great significance for biochronological correlation across the low palaeolatitude belt from the eastern Pacific to the western end of the Tethys.
Three new sections in the Augusta Mountains have yielded a rich faunal succession, whose upper part correlates with the resampled lower portion of the classic Fossil Hill section in the Humboldt Range. In the new biostratigraphical sequence, the scope of the latest middle Anisian Shoshonensis Zone is expanded by the introduction of a new uppermost subdivision, namely the Bulogites mojsvari Subzone, which correlates with the Grossreifling fauna of the western Tethys. The Gymnotoceras weitschati Zone and Gymnotoceras mimetus Zone are introduced at the base of the Late Anisian, in ascending order. The Weitschati Zone, composed of the Billingsites cordeyi and Rieberites transiformis subzones, is approximately equivalent to the ill-defined Trinodosus Zone of the Tethyan realm. Included in the Mimetus Zone are the Dixiceras lawsoni and Marcouxites spinifer subzones. The Gymonotoceras rotelliformis Zone, which was formerly considered as a correlative of the Trinodosus Zone, was previously subdivided into five subzones, mainly on the basis of various species referred to as "Paraceratites". These species, whose respective ranges are shown largely to overlap, are reassigned to the new genera Silberlingia, Ceccaceras, and Brackites. Hence, the number of subdivisions of the Rotelliformis Zone is reduced to two, namely the Brackites vogdesi and Gymnotoceras blakei subzones, in ascending order.
Eleven genera (i.e. Chiratites, Billingsites, Dixieceras, Jenksites, Rieppelites, Rieberites, Marcouxites, Silberlingia, Ceccaceras, Brackites, Oxylongobardites) and 15 species (i.e. Balatonites hexatuberculatus, Chiratites retrospinosus, C. bituberculatus, Billingsites cordeyi, B. escargueli, Gymnotoceras weitschati, G. mimetus, Jenksites flexicostatus, Rieppelites boletzkyi, R. shevyrevi, Rieberites transiformis, Silberlingia praecursor, Ceccaceras stecki, Brackites spinosus, Oxylongobardites acutus) are newly described. "Ceratites" lawsom and "Ceratites" spinifer, as previously described by Smith (1914) and subsequently synonymized by Silberling & Nichols (1982), are recognized as valid species and are assigned to the new genera Dixieceras and Marcouxites, respectively.