MORPHOLOGIC VARIATION AS A DRIVER OF TAXONOMIC RICHNESS: VENERID BIVALVES OF THE INDO-PACIFIC

Studies of modern taxonomic richness look increasingly to the fossil record for information about the formation and development of biodiversity. As a hotspot of modern diversity in marine habitats, the Indo-Pacific has been recognized as a persistent center of origination for the majority of the Cenozoic. In this context, I am investigating the evolution of venerid-bivalve biodiversity throughout the Indo-Pacific with the goal of understanding causal mechanisms for the development of the tremendous molluscan diversity observed in terms of both species richness (i.e. biodiversity) and morphologic variation (i.e. disparity).

I hypothesize that closely constrained biotic interactions, such as the agonistic relationship between venerid bivalves and their predators, shell-drilling naticid gastropods, may cause directional changes in shell shape and form as a heritable phenotypic response. Further, this evolutionary response may contribute to genetic differentiation and promote speciation within preexisting clades. As an initial step, I report here on a preliminary assessment of morphological variation among several Cenozoic venerid genera from the Indo-Pacific in a stratigraphic and geographic context, taking into account frequency of naticid drilling as a potential function of morphological variation.

Digital photographs of fossil material from natural history museums in Europe and the United States were obtained for Neogene specimens from Indonesia, Japan, Malaysia, Australia, New Zealand, and the Philippines. Shape analyses of these bivalve specimens were conducted using biologically significant and homologous landmarks preserved on the shell and also gross shell shape, by means of the overall shell outline. Shell outlines were captured using an automated image recognition process and decomposed into Fourier ellipses and compared across regional geographic scales and through time using Principal Components Analysis. In an initial examination of morphologic attributes of two common venerid genera, Dosinia and Tapes, taxa are randomly distributed throughout shape space with respect to geographic and temporal distributions. However, when predation instances (i.e. drilled specimens) are superimposed on the ordination in shape space, a potential bias in prey selectivity is observed.