TESTING THE ROLE OF BIOTIC INTERACTION IN SHAPING TAXONOMIC, MORPHOLOGICAL, AND FUNCTIONAL DIVERSITY IN ANOMALODESMATAN BIVALVES

Bivalve sister clades Anomalodesmata (Anomalos) and Imparidentia (Imparis) exhibit contrasting diversity trajectories from their divergence in the Ordovician to the present, potentially indicating clade interactions. Within-clade competition may lead to diversity dependence; competition, incumbency, or predation may lead to elevated extinction or depressed origination rates; and environmental perturbations can also drive extinction. In this study I test for the roles of within-clade interactions and negative interactions with its sister clade and predators on the diversity pattern of Anomalos. Anomalos and Imparis dynamics show the most significant correlations in the Mesozoic, with positive correlations of origination of each group with extinction rates of the other. The Mid Triassic marks a dramatic transition as Imparis diversify strongly while Anomalos show only modest diversification through the Mesozoic. Within Anomalos, lineages that have adopted novel food sources or substrata may show a different dynamic from those resembling Imparis, potentially reflecting interactions among Anomalo lineages, between Anomalos and Imparis, with predators, or some combination. More work is needed to identify the exact relationship between these two ecological groups of Anomalos and their respective relationship with Imparis, but the fact that Anomalos achieved net diversification rates in the Cenozoic more than 4 times higher than in the Mesozoic and similar to Cenozoic Imparis suggests evasion from negative interactions. I also expect morphological disparity to reflect interactions or evasion from interactions. In a preliminary morphospace comparison between Jurassic and living Anomalos using shell outlines, Anomalos appear to occupy smaller morphospace volume today than in Jurassic despite almost double the diversity. The Anomalos groups that adopted novel feeding or substratum ecology appear to expand in the morphospace, whereas the rest appear to shrink or stay constant, again suggesting evasion from negative interactions–both within Anomalos and among clades (including predators).