New symphysurinid trilobites from the Cambrian-Ordovician boundary interval in the western United States

The Ibexian (Lower Ordovician) Symphysurina Zone has been subdivided into three subzones (in ascending order): the Symphysurina brevispicata, Symphysurina bulbosa and Symphysurina woosteri subzones. During deliberations over the selection of the GSSP for the base of the Ordovician System the base of the S. bulbosa Subzone was believed to closely approximate the First Appearance Datum (FAD) of the conodont Iapetognathus fluctivagus and the associated restricted occurrence of the cosmopolitan trilobite Jujuyaspsis. This correspondence allowed the approximate position of the Cambrian-Ordovician boundary in platform sequences of Laurentia to be determined on the basis of the endemic trilobite faunas. However, recent work has shown the tripartite division of the Symphysurina Zone to be untenable and, in particular, raised doubt regarding the suitability of the base of the S. bulbosa Subzone as a proxy for the base of the Ordovician System. This study describes four new species from the Cambrian-Ordovician boundary interval that are assigned to the Family Symphysurinidae. Symphysurina ethingtoni sp. nov. and Symphysurina straatmannae sp. nov. are recovered from the uppermost Cambrian. The genus Chasbellus gen. nov. is erected to include punctate symphysurinids that exhibit a concave pygidial border that extends posteriorly as a flat shelf. Two species are formally described: C. milleri (the type species) and C. repetskii, while a third is left in open nomenclature. All species of Chasbellus are restricted to the lowest Ordovician. Collectively, these species allow more confident distinction of the lower (Cambrian) from upper (Ordovician) portions of the Symphysurina Zone than was previously possible.