EVENT BED DEPOSITION IN THE CINCINNATIAN SERIES: IMPLICATIONS FOR TAPHONOMIC PROCESSES AND ASSESSING MICROEVOLUTIONARY CHANGES WITHIN FLEXICALYMENE

Despite intense scientific interest, there is currently little high-resolution data available to understand how organisms respond to multiple episodes of environmental change at time-scales ranging from 10³ to 10⁶ years. The pre-requisites for such a study include exceptional preservation of multiple fossiliferous horizons and a high-resolution stratigraphic framework. Recent stratigraphic analyses of the Cincinnatian Series now provide an exceptionally precise spatial and temporal framework for assessing microevolutionary changes within individual species and species lineages. Initial studies in the area have shown that storm-related episodes of rapid mud deposition smothered populations of live Flexicalymene, preserving unique and substantial samples of contemporary individuals which were alive some 450 million years ago. Thin section analysis has revealed a similiar depositional regime at Mt. Orab, within the Arnheim Formation, with at least seven gradational beds within a 0.46 metre mudstone unit. Differences in size distribution, density, and attitude of the trilobites suggests that the Mt. Orab bed differs from specific behavioral aggregations seen within the "granulosa" bed of the Kope Formation, although both represent contemporaneous populations. Similar "event-bed" deposition occurs throughout the Cincinnatian Series, offering the possibility of repeated sampling of contemporary populations sequentially throughout the Series. Morphometric analysis of these contemporary populations of Flexicalymene will constrain patterns of morphological variability at the population level, providing a firm basis for understanding the morphological variation evident within Flexicalymene from the Cincinnatian, and its relationship to multiple episodes of environmental change within a sequence stratigraphic framework.