Measuring recurrence of marine biotic gradients: A case study from the Pennsylvanian-Permian midcontinent

The aim of this study was to determine whether biotic associations of Pennsylvanian-Permian brachiopods and bivalves from the northern Midcontinent differ in their degree of recurrence through time. The study interval includes 2.5 Myr that can be divided into 5 full and 2 partial composite depositional sequences separated by subaerial unconformities. These stratigraphic packages represent replicate natural experiments in establishing the benthic marine ecosystem of the basin. Based on cluster and ordination analyses, two discrete biofacies can be recognized-one dominated by brachiopods and the other by bivalves. Within each of these, environmental gradients can be recognized. The brachiopod gradient is interpreted to reflect the degree of water-column oxygenation, whereas the bivalve gradient is interpreted to reflect the transition from restricted to open-marine conditions. Comparison of measured recurrence with randomized data indicates that the ecological segregation of the two biofacies is maintained to a significant degree through the succession of depositional sequences in the study interval. In contrast, the gradients within each biofacies, although recognizable, are not maintained rigidly from sequence to sequence. There is also no significant difference in gradient recurrence between the two biofacies. These results imply that there is no need to call upon strong interspecific interactions to maintain the structure of these paleocommunities through time.