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

史密斯晚期的灭绝代表了早三叠世的一个重大事件。该事件通常对应于连续的两个，可能是三个连续的不太多样化的世界性菊花组合，当存在时，它们提供了强大的生物地层框架和不同空间尺度的精确相关性。在美国西部盆地，已知的最新史密斯分类群很少见，直到现在，只有内华达州东北部有记录。基于这些受限制的盆地事件，假设存在一个代表最新史密森的区域带，但没有得到证实，因为尚未从内华达州以外报告有代表性的分类群。在这里，我们记录了来自犹他州北部遥远地区的两个新的菊石组合，覆盖在晚期的史密斯 Anasibirites 上床并以Xenoceltites subevolutus和Pseudosageceras augustum的明确共存为特征。因此，美国西部盆地中存在最新的史密斯带得到证实，有助于识别史密斯/斯帕斯边界和盆地内相关性。该带还与从南特提斯盆地确认的最新史密斯带相关联。此外，这些新数据支持在史密斯晚期的大部分时间里观察到的Xenoceltites subevolutus 的其他出现。