Temporal persistence and stability of Cambrian biofacies: Sunwaptan (Upper Cambrian) trilobite faunas of North America
Abstract During the Upper Cambrian Sunwaptan Stage (= “Ptychaspid Biomere”), trilobite faunas diversified following a mass extinction at the base of the unit. During and immediately after the extinction interval, a small number of low diversity trilobite biofacies were spread widely over the shelf. Turnover at all taxonomic levels was brisk and, in all facies, there was a sequence of rapid biofacies replacements. By the end of the Sunwaptan, a well-developed spectrum of environmentally-circumscribed trilobite biofacies was developed in carbonate depositional systems and tracking of lithofacies by biofacies can be demonstrated. However, rapid turnover still took place at the species level, so persistence and stability of biofacies was expressed at the clade level. A preliminary assessment of other Sunwaptan faunal elements, including articulate and inarticulate brachiopods, suggests that they were characterized by more persistent, stable assemblages, even at the species level. The pattern of stability of Cambrian trilobite biofacies stands in marked contrast to the species level, coordinated stasis documented for the articulate brachiopod-rich biofacies of the Devonian of New York State. The difference in behavior of these biofacies may simply be an expression of differences in the extinction and speciation rates of component taxa. Biofacies assembled from volatile taxa, such as Cambrian trilobites, will inevitably display rapid turnover, whereas coordinated stasis is likely to be characteristic of less volatile groups, such as articulate brachiopods or bivalves. The extent of biofacies stability will therefore be controlled ultimately by factors influencing evolutionary rates, such as behavioral complexity, niche breadth, population size, and dispersal ability.