Latest Smithian (Early Triassic) ammonoid assemblages in Utah (western USA basin) and their implications for regional biostratigraphy, biogeography and placement of the Smithian/Spathian boundary

The late Smithian extinction represents a major event within the Early Triassic. This generally corresponds to succession of two, possibly three successively less diverse, cosmopolitan ammonoid assemblages, which when present, provide robust biostratigraphic framework and precise correlations at different spatial scales. In western USA basin, known occurrences latest taxa are rare until now, have only been documented from northeastern Nevada. Based on these restricted basinal occurrences, regional zone representing was postulated but not corroborated, as representative had yet reported outside Here we document two new assemblages distant localities in northern Utah, overlying Anasibirites beds characterized by unambiguous co-occurrence Xenoceltites subevolutus Pseudosageceras augustum. existence basin is therefore validated, facilitating identification Smithian/Spathian boundary intra-basin correlation. also correlates with recognized southern Tethyan basins. Additionally, data support other observed throughout most Smithian.