Inarticulate brachiopods of the late Marjumiid and Pterocephaliid biomeres (late Middle-early Late Cambrian) of west-central Utah and east-central Nevada, United States

Iterative episodes of faunal turnover bounding periods of radiation, called biomeres, have been documented in polymeroid trilobites of North America throughout the Cambrian. Although many hypotheses, including changes in sea level, temperature, oxygen content, and biofacies shifts, have been proposed to explain the turnovers at the biomere boundaries, the causes remain uncertain. The goal of this dissertation is to refine possible causes of the faunal turnovers by studying inarticulate brachiopods through two Middle-Late Cambrian biomeres (Marjumiid and Pterocephaliid) in west-central Nevada and east-central Utah.; The area around a fault-controlled trough that developed in the Cambrian, called the House Embayment, was chosen for this study because it provides an ideal setting for comparing faunal changes in polymeroid trilobites and inarticulate brachiopods. Ten stratigraphic sections were measured and sampled from different points in the House Embayment representing different depositional environments. The lithologies present at all sections were grouped into 11 Lithofacies and four Lithofacies Complexes to determine the environmental distributions of the brachiopods and to compare faunal and environmental changes.; The brachiopod fauna from this study includes 20 genera and 26 species. The pattern of faunal change in the inarticulate brachiopods appears to follow that of the polymeroid trilobites across the biomere boundaries. Compared to the polymeroid trilobites, the inarticulate brachiopods have longer stratigraphic distributions and wider geographic ranges. However, most species of inarticulate brachiopods and polymeroid trilobites go extinct at the end of each biomere, and the coincidence of faunal turnover in both groups suggests a common causal factor in the environment.; This study shows no consistent lithofacies change near the biomeres boundaries, suggesting that environmental shifts did not lead to the extinctions. Brachiopod biofacies cannot be distinguished; therefore the hypothesis of shifting biofacies as a cause for the extinctions is not supported. In addition, because brachiopods were living in colder environments, an incursion of cold water onto the shelf could not cause the extinction of so many species seemingly tolerant of this condition. As a result, a change in the concentration or distribution of oxygen in the water column remains the most plausible cause for the faunal turnover in both groups at both biomere boundaries.