ARE SECULAR TRENDS IN TIERING, MOTILITY, AND PREDATION REAL? QUANTIFYING AND TESTING CHANGES IN ECOSPACE USE BETWEEN THE PALEOZOIC AND CENOZOIC

Organisms segregate into different sections of the "multidimensional hypervolume" of ecospace in order to coexist in the same environment. To quantify changes in the use of ecospace between level-bottom, marine paleocommunities from the middle Paleozoic and late Cenozoic, we assigned every genus in a database of middle Paleozoic and late Cenozoic fossil assemblages to an ecotype category in theoretical ecospace based on tiering, motility level, and feeding type. This multidimensional ecospace is "theoretical" because it includes all combinations of ecologic parameters, not just those that are occupied, and allows for quantitative tests of changes in ecosystem structure through time. Paleozoic assemblages are numerically dominated by the remains of surface-dwelling epifauna, whereas Cenozoic assemblages boast an abundant infauna, both shallow and deep. Paleozoic assemblages are dominated by non-motile forms such as brachiopods and bryozoans, but these ecotypes had decreased in importance by the Cenozoic as facultatively motile bivalves rose to dominance. Facultative motility allows the bivalves to reorient after disturbance and escape predators. Non-motile, unattached recliners were virtually eliminated by the late Cenozoic, presumably affected by amensalism from "biological bulldozers". Suspension feeders dominated in both the Paleozoic and Cenozoic, but predators increased in local relative abundance by the late Cenozoic. We also modeled the effects of the preferential dissolution of aragonitic skeletons on Paleozoic assemblages by increasing the relative abundance of aragonitic taxa using techniques developed in Bush and Bambach (Journal of Geology, in press). The major changes in ecotypes discussed above were unaffected, although the fates of several other ecotypes were ambiguous and depended on the strength of the aragonite dissolution bias. For example, grazers and fully, regularly motile ecotypes may or may not have changed in abundance. Only a limited number of ecotypes were common at any time, but the dominant ecotypes changed between eras, in part due to biological feedbacks related to the Mesozoic Marine Revolution. Cenozoic ecosystems boast a greater abundance of ecotypes with high-energy lifestyles, consistent with increased food availability.