A MASSIVE CHANGE IN THEORETICAL ECOSPACE UTILIZATION? BIOMASS VERSUS RELATIVE ABUNDANCE IN THE STUDY OF LOCAL FOSSIL ASSEMBLAGES

Marine benthic ecosystems have changed greatly since the Paleozoic, with documented increases in motility, infaunalization, and predation. Most studies of these paleoecological trends have focused on some measure of relative abundance or taxonomic richness that is based on counts of taxa, occurrences, or individuals. However, some studies have used measures of biomass and energy use, which vary widely among animals. Ecological trends based on these measures can differ greatly from ones based on simple counts. For example, Payne et al. (2014, Proc. Royal Soc. B) showed that bivalves dominated brachiopods in terms of energy use during much of the Paleozoic, even though brachiopods are generally considered to be more abundant and diverse at that time. Many studies of biomass and energy use, such as that of Payne et al., focused on broad-scale global or regional trends, and a single estimate of biomass was applied to all individuals in each species or genus. Here, we concentrate instead on local-scale patterns and evaluate the paleoecology of individual fossil assemblages using estimates of biomass and energy use. Given the local focus, biomass is estimated using body size measurements of individual specimens in our collections, such that body size variation among individuals in a species is considered. We examined bulk samples from the Pleistocene Fort Thompson Formation of Florida and the Pennsylvanian Breathitt Formation of Kentucky, which is notable for preservation of aragonitic-shelled animals as shells, not molds. We evaluated the ecologic composition of the faunas using the three-dimensional ecospace model of Bush, Bambach, and Daley (2007, Paleobiology) and compared the apparent ecological importance of different modes of life based on counts, biomass, and energy use. We also assessed the differences between our Paleozoic and Cenozoic paleocommunities with these measures and investigate Payne et al.’s finding at the level of individual ecosystems.