TAXONOMIC DIVERSITY OF DEEP SUBTIDAL, SOFT-SUBSTRATE “COMMUNITIES” DURING THE CAMBRIAN THROUGH DEVONIAN

Twenty-five years ago, Richard Bambach’s classic study established, among other results, that the median alpha-diversity of open marine fossil assemblages increased 50% during the Paleozoic. Since then, few studies have focused on the ecology of fossil assemblages in individual habitats over large time scales. This is unfortunate because focusing on a single habitat allows one to better interpret the evolution and paleoecology of species while simultaneously controlling for environmental and taphonomical characteristics.

Using a literature-based survey of well-preserved fossil collections, this study investigates the taxonomic diversity of assemblages in deep subtidal, soft-substrate settings during the Cambrian through Devonian. This habitat was selected for its high preservation potential and relative environmental stability. Moreover, taxa within this habitat underwent dramatic evolutionary changes during this time interval, allowing for tests of their ecological consequences. The database consists of more than 300 collections that have been standardized for variation in taphonomy, sampling effort, and environmental characteristics, including oxygenation, depth, and sedimentation. Approximately half of these collections contain information on species abundance, enabling more robust standardization of sampling effort and ecologically more informative measures of diversity.

Results demonstrate that mean alpha-diversity in this habitat increased significantly during both the Ordovician and the Silurian. The prior diversification was accompanied by significant increases in the number of classes present within collections, while the latter resulted primarily from species-level diversification within already-established classes. This pattern is supported by the median, rarefaction, and Shannon-Wiener diversity measures. Despite significant changes in alpha-diversity and class-level composition, evenness (or equitability) remained high and relatively constant throughout this interval, implying that the diversification of species within these habitats was primarily accompanied by either increased population densities, equable diversification of species in all abundance classes, or both.