The ammonites described in the present Memoir were collected from the Sunrise Formation in the Gabbs Valley Range, west-central Nevada and are of latest Triassic (late Rhaetian) and early Jurassic (Hettangian) age. The fauna is composed of 50 species allocated to 26 genera. Nine of those species are described as new.
Documentation of the stratigraphic distribution of the species permits the recognition of 10 biochronologic subdivisions which can be correlated with coeval faunas elsewhere in the Americas as well as with the European ammonoid succession.
The Nevada fauna has tethyan affinities. Tethyan ammonoid faunas are well known from the Lower Lias of the Eastern Alps, but they occur in condensed stratigraphie sequences inhibiting an understanding of the range relationships of many ammonite species and genera from that region. Consequently, the Nevada record is significant because it provides critically needed documentation of Hettangian ammonoid ranges from the Tethyan Realm. Because the fauna is a diverse Tethyan one occurring in an expanded stratigraphic sequence, it enhances substantially our understanding of the terminal Triassic extinction event and ensuing basal Jurassic ammonoid diversification.