SMALL SHELLY FOSSILS FROM THE DYERAN-DELAMARAN (“LOWER”-“MIDDLE” CAMBRIAN) BOUNDARY INTERVAL OF THE PIOCHE-CALIENTE REGION, NEVADA, USA

A thick succession of Cambrian rocks is exposed in the southern Great Basin of the southwestern U.S.A., and the lithostratigraphy and biostratigraphy (particularly that of trilobites) of this region have long been studied. Less attention has been paid to the so-called small shelly fossils, a polyphyletic assemblage of shells and sclerites typically preserved by secondary phosphatization. We studied 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 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 carbonate samples yielded abundant small shelly fossils, including pelagiellid molluscs, chancelloriid sclerites, and hyoliths, along with trilobites, linguliform brachiopods, and rare echinoderm ossicles and spicules of hexactinellid and heteractinid sponges. Pelagiellids are abundant in the upper Dyeran, and include at least three species (Costipelagiella nevadense with radial costae, Pelagiella sp. with a smooth external surface, and a very loosely coiled form), but they are entirely absent from Delamaran samples. Their local disappearance is approximately coeval with the extinction of olenelloid trilobites, although pelagiellids are known from younger strata elsewhere in the world. In contrast, hyoliths and chancelloriids range through the entire studied interval. Several samples yielded large samples of complete chancelloriid sclerites, allowing systematic treatment. Chancelloriid sclerites with central rays (Chancelloria sensu lato) dominate most samples from both the Dyeran Combined Metals Member and the Delamaran Comet Shale Member, but each member seems to be characterized by a different species. Most chancelloriid sclerites from the overlying Susan Duster Member, in contrast, can be ascribed to Archiasterella. In combination with new trilobite biostratigraphic, sequence stratigraphic, and chemostratigraphic data, these observations will help refine the Cambrian timescale during this critical interval of metazoan history.