Smithian ammonoid faunas from northeastern Nevada: implications for Early Triassic biostratigraphy and correlation within the western USA basin

Intensive sampling of the lower portion of the Thaynes Group within the Palomino Ridge area (northeastern Nevada) yielded abundant and well-preserved Smithian (Early Triassic) ammonoid faunas. Ammonoid taxonomy and a detailed biostratigraphy for this locality are reported herein. One new genus (Palominoceras) and one new species (?Pseudosageceras bullatum) are described. Additionally, based on new data from Palomino Ridge and previous data from neighboring localities in Utah, we porvide here the first quantitative Smithian ammonoid biochronological scheme for the western US basin. This new zonation is based on the Unitary Associations (UA) method. The biochronical sequence comprises five unitary association zones that can be correlated with other localities from the Northern Indian Margin (Salt Range, Pakistan, Spiti, northern India and Tulong, South Tibet). Three unitary association zone (UAZ1, UAZ2 and UAZ3) are defined for the early Smithian, one (UAZ4) spans the entire middle Smithian and one (UAZ5) signifies the first part of the late Smithian. Finally, a hypothetical UAZ6 would represent the second part of the late Smithian.