TRILOBITE FAUNAS ACROSS THE BASE OF THE EOCONODONTUS NOTCHPEAKENSIS CONODONT SUBZONE, A PROPOSED BASE FOR GLOBAL CAMBRIAN STAGE 10

Although a GSSP for the base of the uppermost global stage of the Cambrian System, Cambrian Stage 10, is yet to be selected, two primary candidate horizons have emerged: the FAD of the agnostoid arthropod Lotagnostus americanus and the FAD of the conodont Eoconodontus notchpeakensis. Suitability of the former has been challenged recently due to the broad taxonomic species concept applied for L americanus, while extensive evidence of the utility of the latter has accumulated from several regions in North America. To that end, we have more fully documented the trilobite faunas associated with this boundary through intensive sampling across the base of the Eoconodontus Zone in Utah and central Texas.

We collected trilobite data within a highly resolved conodont zonation from three measured sections in the Notch Peak Formation in Utah: Sneakover Pass, Steamboat Pass, and Lava Dam 5 East. The richest collections came from Sneakover Pass where faunas in the lower portion of the Red Tops Member are dominated by Eurekia, Prosaukia, Saukia, and Illaenurus, specifically I. montanensis, a species restricted to the upper part of the Illaenurus trilobite Zone. Saukiid species present in collections from the underlying Hellnmaria Member at Steamboat Pass include Prosaukia pyrene and Saukiella pepinensis? from 6.5m and 5.3m below the FAD of E. notchpeakensis, respectively, allowing correlation of the interval sampled through the Red Tops Member in this study with the lower part of the Saukiella junia Subzone of the Saukia Zone in central Texas.

Although there is no sharp turnover in the trilobite fauna associated with the FAD of E. notchpeakensis in Utah or Texas, a number of species that span this horizon have relatively short ranges, and would allow approximate placement of the proposed stage boundary. In Utah, these include several species originally described from the Bowmania lassieae Fauna of Nevada, specifically: Euptychaspis dougali, Eurekia rintintini, and Cherrycreekia benjii. Bayfieldia binodosa displays a similar range in Texas. Additionally, a number of species range upward to a short distance below the boundary, such as Proricephalus scapane, Prosaukia pyrene, and Eurekia granulosa in one or both areas. Others (e.g., Stenopilus latus and “Bayfieldia simata”) have FAD’s a short distance above the proposed boundary.