SMALL SHELLY FOSSIL BIOSTRATIGRAPHY OF CAMBRIAN STAGE 4 (DYERAN–DELAMARAN BOUNDARY) OF THE PIOCHE–CALIENTE REGION, NEVADA, USA

Small shelly fossils, a polyphyletic assemblage of shells and sclerites often preserved by secondary phosphatization, have been extensively studied from the basal Cambrian. Much less is known about the history of these groups after the mid-early Cambrian, however, due to some combination of a genuine extinction or decline in these animals, a secular decrease in secondary phosphatization, and a dearth of paleontological work. We studied a series of nine sections of the Pioche Formation in the Pioche–Caliente region of east-central Nevada that span the upper Dyeran (uppermost traditional “lower” Cambrian of Laurentia) to lower Delamaran (lowermost traditional “middle” Cambrian) stages, corresponding to upper Stage 4 to basal Wuliuan in the global chronostratigraphy. Acid maceration of carbonate samples yielded abundant fossils; linguliform brachiopods and silicified trilobites are widespread throughout the studied interval, and a variety of other shelly fossils are preserved as internal molds or external coatings of calcium phosphate or phyllosilicates. Fossils from the latest Dyeran Combined Metals Member include abundant pelagiellid molluscs (of at least two species), hyoliths, and chancelloriid sclerites (including four species), along with less common helcionelloid molluscs, sponge spicules, echinoderm ossicles, and probable embryo cases (Archaeooides). The overlying basal Delamaran Comet Shale Member yielded a more restricted fauna of hyoliths and chancelloriids (three species). The overlying Susan Duster Limestone Member hosts chancelloriids (two species), hyoliths, helcionelloid molluscs, and cambroclaves. There is thus a consistent biostratigraphic pattern seen across our study area, including a major turnover at the Dyeran–Delamaran boundary coeval with the extinction of olenelloid trilobites. In general, the Pioche Formation shows relatively low levels of diversity of small shelly fossils compared with assemblages from elsewhere in the world, consistent with previous results from western Laurentia; given the abundance and often excellent preservation of fossils in our samples, this is unlikely to be a taphonomic artifact. Our results are also consistent with the global pattern of a decline in scleritome-bearing and tube-dwelling small shelly fossils by the mid-Cambrian.