DECLINING SKELETAL ANIMAL ABUNDANCE DURING AND AFTER THE ARCHAEOCYATH EXTINCTION IN THE WESTERN UNITED STATES

The archaeocyaths, the first calcifying sponges of the early Cambrian, provided the skeletal frameworks to early metazoan reef communities. During this interval, these reefs are thought to have driven evolutionary abundance and diversity both in reefs and in the benthic communities surrounding them. The archaeocyaths disappeared within the early Cambrian in an extinction event that has been documented globally, and here, we present new data to suggest that this extinction is preserved in the upper Harkless Formation of western Nevada. To determine if and how benthic communities were affected during the extinction interval, samples from non-reef environments were collected from the Poleta, Harkless and Mule Springs formations in western Nevada. The grain-solid method was employed to point count 253 thin sections spanning from the abundant archaeocyathan reefs in the lower Poleta Formation into the sparse post-extinction facies of the Mule Springs Formation. Pre-extinction samples contain an average of 12.4% skeletal material, ranging from 0-25%, while post-extinction samples contain an average of 1.8% skeletal material, with a range from 0-13%. While the samples taken from before the archaeocyath disappearance contain an abundant assemblage of echinoderms, brachiopods, trilobites, and Salterella, skeletal carbonate production declines across the extinction interval to facies dominated by ooids and oncoids, with some rare trilobites and echinoderms. Overall, as the archaeocyathan reef environments of the early Cambrian disappeared, a simultaneous decrease in skeletal abundance is observed in benthic communities in the western United States.