LINGULIFORM BRACHIOPODS ACROSS THE STEPTOEAN/SUNWAPTAN (LATE CAMBRIAN) “BIOMERE” BOUNDARY IN THE GREAT BASIN, USA

The North American Steptoean/Sunwaptan stage boundary is defined at a trilobite extinction event that is followed by the appearance of globally distributed taxa. The repeated family-level trilobite extinctions of the Late Cambrian to earliest Ordovician can be hypothesized to represent a “clearing of the way” for the Great Ordovician Biodiversification Event (GOBE), or alternatively as a temporary diversity lull separating the Cambrian Explosion from the continued diversification of the GOBE. We test the hypothesis that the effects of these extinction events can be detected in other major groups of the Cambrian Fauna. Detailed collections from below, across, and above the Steptoean/Sunwaptan boundary document the ranges of linguliform brachiopods within this interval in western Utah (Orr Formation) and eastern central Nevada (Dunderberg Shale and lower Windfall Formation), including at the stratotype Steptoean section near McGill, Nevada. Mid–late Steptoean strata yield abundant Apsotreta expansa, followed by equally abundant Linnarssonella girtyi, two closely related taxa that are endemic to North America but widespread in strata of this age within North America. Within uppermost Steptoean strata, the appearance of genera and species (Quadrisonia, Zhanatella) with ties to Kazakhstan and Gondwana suggest that events that triggered the trilobite extinctions and subsequent migrations also resulted in the same consequences for linguliform brachiopods in the Laurentian shelf–slope environment.

Additionally, while Linnarssonella girtyi and Apsotreta expansa have been documented widely and used for biostratigraphic zonation in central North America, we present the first detailed correlations of the Apsotreta expansa and Linnarssonella Zones from central Laurentia (e.g. Missouri, Texas) to the Great Basin. Late Steptoean/Early Sunwaptan strata are devoid of conodonts that are useful for correlation, and linguliform brachiopod-based biostratigraphy is an overlooked micropaleontological tool for understanding the complexities of Late Cambrian correlation within North America.