Trilobite alpha diversity and the reorganization of Ordovician benthic marine communities

The Ordovician saw the transformation of marine benthic communities from the trilobite-based Cambrian Fauna to the brachiopod-dominated Paleozoic Fauna. An evaluation of the changing importance of trilobites during the Ordovician can be made from accurate assessments of taxonomic richness in various habitats. Here we present a new compilation of trilobite alpha diversity based on field collections and survey of the literature. The data indicate that trilobite species richness within nearshore, shallow subtidal, carbonate buildup and deep subtidal shelf environments was essentially constant between the Late Cambrian and the Late Ordovician. The alpha diversity patterns do not support the notion that trilobites became displaced from inner shelf environments during the Ordovician. Rather, the data are consistent with a decline in relative importance of the group through dilution as newly radiating invertebrate groups entered Ordovician paleocommunities. They also imply that direct interactions between elements of the Cambrian and Paleozoic faunas were not involved in the Ordovician reorganization of paleocommunities. Like many other major faunal transitions during the Phanerozoic, the Ordovician radiations appear to have been essentially non-competitive in nature.