TESTING SELECTIVE EXTINCTION AND BIOTIC INTERACTION IN THE CARVING OF MORPHOSPACE USING ANOMALODESMATAN BIVALVES

Environmental calamities and biotic interactions act on origination- and extinction-rates to push evolutionary trajectories away from Brownian motion. Morphospace dynamics may vary across morphospace regions in response to biotic and environmental drivers, changing aspects of morphospace occupation (such as volume, nearest-neighbor distances, and distances from the centroid). Extinction events often drive discordant decline among the three major evolutionary currencies— taxonomic richness, functional diversity, and morphological disparity of surviving taxa, representing different patterns of selective extinction. Anomalodesmata (“Anomalos”), a subclass of Bivalvia today showing low taxonomic richness and high functional diversity, represents the cumulative result of origination and extinction owing to the interplay of environmental and biotic factors. Today, Anomalos occupy some of the most specialized niches such as carnivory and crypt-dwelling. Since their first occurrence in the Early Ordovician, Anomalos have survived five major extinction events—most notably the end-Permian (PT) and the end-Cretaceous (KPg) ones—each of which eliminated >50% of Anomalo genera. Using shell outline as a measure of morphology, I quantify the decline and recovery associated with PT and KPg extinction events in various aspects of the morphospace. Here I test for selective extinction and convergent evolution in Anomalo shell-shape space, sampling lineages from the mid-Mesozoic, the Recent, and time intervals before and after PT and KPg (plus their Early Ordovician start). Extinction events tend to select against strong ornamentation, while biotic interactions may define fitness peaks in the morphospace driving convergent evolution. Excluding Clavagelloidea owing to a lack of homologous features, 37 genera of Jurassic Anomalos unexpectedly occupy more volume in the morphospace than 73 genera of Recent Anomalos. Cenozoic Anomalo lineages show little evidence of diversity-dependent evolution either among themselves or with respect to Imparidentia, their putative competitor. The trajectories of richness and of disparity are evidently decoupled between the mid-Jurassic and today, and this study analyses the roles of selective extinction and biotic interaction in this decoupling.