THE ABUNDANCE OF ANIMALS IN LOWER CAMBRIAN CARBONATES OF THE WESTERN UNITED STATES: THE EFFECTS OF THE ARCHAEOCYATHAN EXTINCTION ON BENTHIC COMMUNITIES

In the early Cambrian Period, the archaeocyaths, an early calcifying sponge, became the dominant organisms to produce the first biotic reefs with skeletally secreted frameworks. Reefs have been shown to be a major driver of evolutionary diversity and abundance during the early Cambrian interval but these archaeocyathan reefs were short-lived, becoming extinct before the close of the early Cambrian. In strata of the western United States, archaeocyathan reefs are well known through the lower Cambrian Poleta and Harkless formations, with their disappearance constrained to sometime after the Harkless reefs. Although this disappearance of archaeocyaths has been documented globally, the impacts of this extinction on other non-reef benthic organisms are less well known. Samples from reef-adjacent environments in the Harkless and Mule Springs formations that pre- and post-date the extinction of archaeocyathan reefs were collected at Palmetto Mountain, Jackson Mountain, and near Gold Point, Nevada. These samples were point counted using the grain solid method to glean insights into the nature of skeletal carbonate production outside of reef settings before and after the demise of the earliest animal reefs. Prior to the extinction, samples range from 0-25% (average 12.4%) skeletal material, which changes rapidly in the uppermost Harkless and lower Mule Springs (ranges from 0-13% skeletal material, average 1.8%). The fossiliferous grainstone facies in the pre-extinction Harkless, which contain trilobites, echinoderms, brachiopods and Salterella, are replaced by oolitic and oncoidal non-skeletal facies in the post-extinction interval, and skeletal diversity becomes dominated by trilobites and echinoderms only. As a whole, it appears that in addition to the decline of skeletal organisms that inhabited reef settings, organisms in other benthic ecosystems also show a loss of diversity and abundance across the early Cambrian extinction in the western United States.