QUANTIFYING ECOLOGICAL DISPARITY: COMPARATIVE PALEOECOLOGY OF ORDOVICIAN AND RECENT MARINE ASSEMBLAGES

We present a versatile, new method for comparing ecological entities, ranging from individuals and lineages to entire communities. The framework consists of more than fifty logically-independent ecological characters that are universally applicable to extant and extinct organisms and theoretically independent of phylogenetic relatedness. Most characters, including aspects of microhabitat, mobility, diet, foraging habit, body size, and abundance can be coded unambiguously based on actual observation, functional morphology, behavior (ichnology), environmental distributions, and comparison with living analogues. In some cases, sufficient information is available to code additional characters, such as geographic range, metabolic rate, and aspects of larval and developmental ecology. Simulations demonstrate that the method is relatively insensitive to sampling variation and resilient to errors or ambiguity in coding of ecological characters.

A case study compares the ecological structure of deep-subtidal, soft-substratum assemblages spanning approximately 450 million years. Late Ordovician (Type Cincinnatian) samples represent tropical, epeiric settings, and are dominated by molluscs, brachiopods and trilobites. Recent samples range from the Gulf of Carpentaria (Australia) Indo-Pacific, tropical, epeiric sea through the U.S. Western Atlantic continental shelf, and are dominated by polychaetes, bivalves, and various echinoderms and crustaceans. Although these samples contain few orders and even classes in common, they are comparable in numerous ecological characters, including aspects of mobility, stratification (tiering), feeding microhabitat, and several substratum relationships. This similarity is even more pronounced when comparisons are restricted to the easily fossilized fraction. However, significant ecological differences remain, even after standardization to correct taphonomic and sample-size-related biases. Among others, Ordovician samples generally contain greater levels of sessile, filter-feeding, and epibenthic habits. Furthermore, Recent assemblages utilize a greater range of ecospace than Ordovician assemblages in comparable marine habitats. This increase is consistent with several ecological hypotheses, including passive and biotically-driven models of ecospace diversification.