WEIGHING ENVIRONMENTAL AND BIOTIC FACTORS ON THE ECOMORPHOLOGICAL EVOLUTION AND SPATIAL DISTRIBUTION OF MARINE BIVALVES

Both physical environmental factors and biotic interactions govern the types of organisms that co-occur, but their relative effects on the spatial distribution of marine animals remains poorly understood. Using a set of closely related marine bivalves with a rich fossil record (Chione), I examined the relative effects of geographic occurrence and species co-occurrence on ecologically-relevant shape change with a Bayesian multilevel model.

Closely related species are expected to share similar ecological abilities, which might promote ecomorphological change driven by competition for similar resources. However, present-day populations of Chione species show parallel changes in body size, shell inflation, commarginal outline, and pallial line across geographic regions of co-occurrence. The gain or loss of congeneric species across regions does not predict within- or among-region trait divergences. Thus, comparative biogeography of extant populations suggests that the ecomorphology of these species tracks underlying spatial variation in the environment and not potential interactions.

At the lineage-level, trajectories of trait evolution among Chione species from the Miocene to the Recent also suggest a predominant role for environmental selection. Eastern Pacific species exhibit stasis in burrowing ecology from their origination to the present day, and extant species from the Western Atlantic show convergent trait trajectories both towards each other and the ecomorphological forms previously occupied by extinct relatives.

These population- and lineage-level results imply a lesser role for intraclade competition in shaping shell morphology. Instead, environmental selection, whether from abiotic or "extra-cladal" biotic pressures (e.g., predation), appears to be the dominant factor shaping ecological aspects of form within this marine bivalve clade. Combined biogeographic, morphometric, and paleontological analysis provides a powerful approach for evaluating the long-held hypothesis of character displacement against alternatives that may prove to be more important in structuring marine clade morphology.