CHANCELLORIID SCLERITES FROM THE DYERAN–DELAMARAN (“LOWER”–“MIDDLE” CAMBRIAN) BOUNDARY INTERVAL OF THE PIOCHE–CALIENTE REGION, NEVADA, USA

Chancelloriids are a poorly understood group of problematic Cambrian metazoans. Complete specimens preserved in Burgess Shale-type assemblages show they were sessile, radially symmetric, club-shaped organisms covered with sclerites in the form of little rosettes of spines, so that the animal looked something like a barrel cactus. Isolated sclerites are common components of Cambrian shelly assemblages, but have been the subject of relatively little study because of the difficulty of diagnosing species from single sclerites. Recent work suggests, however, that that careful analysis of large samples of sclerites allows different species to be reliably distinguished. We studied chancelloriid sclerites from a series of nine sections spanning the upper Dyeran (uppermost traditional “lower” Cambrian of Laurentia) to lower Delamaran (lowermost traditional “middle” Cambrian) stages from the Pioche–Caliente region of southeastern Nevada. Samples were collected from the Combined Metals, Comet Shale, and Susan Duster members of the Pioche Formation on a bed-by-bed basis with sub-meter scale resolution; acid maceration of carbonates yielded abundant chancelloriid sclerites. A Chancelloria-like form, having a single central ray surrounded by a circlet of 5–8 marginal rays, is widespread in the Dyeran Combined Metals Member. In a few sections, this form is replaced near the top of the Combined Metals Member by the chancelloriid Archiasterella, whose sclerites have one large recurved ray only partly surrounded by smaller rays. The basal Comet Shale, which preserves a species-poor assemblage following the extinction of the olenelloid trilobites, hosts two morphotypes: a four-rayed Archiasterella and an unusual new form that shares features with both Archiasterella and Chancelloria-like forms. Two further chancelloriids, a second type of four-rayed Archiasterella and a different Chancelloria-like form, characterize the Susan Duster Member. These results provide new support for the taxonomic tractability and biostratigraphic utility of chancelloriid sclerites, and they highlight that small shelly fossils, long an icon of the radiation of animals in the earliest Cambrian, also help document the effects of biotic crises later in the period.