SMALL SHELLY FOSSILS FROM THE DYERAN AND DELAMARAN (LOWER AND MIDDLE CAMBRIAN) OF NEVADA, USA

A rich record of the diversification of biomineralizing animals during the Cambrian radiation may be found in the so-called small shelly fossils, a polyphyletic assemblage of shells and sclerites. Such fossils are secondarily phosphatized or primarily phosphatic, and have been reported in Cambrian carbonates from around the world. A thick succession of Cambrian rocks is exposed in the different ranges of the Basin and Range province of southwestern North America, allowing examination of Cambrian strata spanning shelf-edge to peritidal paleoenvironments. Although the lithostratigraphy and biostratigraphy (particularly that of trilobites) of this region has long been the subject of study, its small shelly fossils remain relatively poorly known, with most previous work being limited to either description of isolated spot samples or passing mention in the context of other research. Herein we report preliminary results from the study of acid residues from a series of measured sections in southern Nevada, with a particular focus on the Dyeran and Delamaran stages (corresponding to the top of the traditional “lower” Cambrian and base of the traditional “middle” Cambrian of Laurentia). A variety of different fossils was recovered; particularly common groups include hyoliths, chancelloriids, helcionelloid molluscs, and paiutiids. These occurrences improve our understanding of the early diversification of these groups and, when combined with novel trilobite biostratigraphic, sequence stratigraphic, and chemostratigraphic data, offer the potential to greatly refine the Cambrian timescale during this critical interval of metazoan history.