BIVALVE PHYLOGENY AND THE HIERARCHICAL NESTING OF MORPHOLOGICAL NETWORKS IN BIVALVE CLADES

Morphometric descriptions of bivalve shells contain information on phylogenetic relatedness at low taxonomic levels. The shell as an external skeleton is bound intimately to the growth and development, soft-anatomy, and functional morphology of the entire organism. Conchological anatomy is therefore a valid repository of homologous and synapomorphic characters, as well as ontogenetic and functional information. Quantitative analysis of conchological characters has the ability to unite these aspects of the shell into a network of character covariance, or "morphometric bauplan." This covariance network reflects the pattern of morphological integration of the shell. Furthermore, based on recent phylogenetic analyses at low taxonomic ranks, we should expect morphometric networks to be nested in a hierarchy representative of phylogenetic branching patterns. The pertinent questions then become: 1) Can information of evolutionary importance be gained from morphometric data on bivalve shells, and 2) if yes, how deeply is the information nested in the phylogenetic tree of the Bivalvia? The difficulties encountered when trying to derive higher-level phylogenies using conchological characters suggest that there are limits to the extent of network nesting. This paper explores these issues with analyses of taxa belonging to the venerid subfamilies Chioninae and Venerinae.

The Chioninae and Venerinae are distributed globally and date back to the Early Oligocene and Middle Eocene, respectively. Species belonging to the families are extremely similar, and share general features of ecology, biogeography, sculpture and other conchological characters, and cardinal tooth morphology. The single most consistent discriminating character is the presence of an anterior lateral hinge tooth in the venerine left valve. It is hypothesized that Chioninae is polyphyletic and comprises several lineages derived from venerine ancestors. This hypothesis will be tested cladistically and compared to the hierarchy derived from the morphometry of both subfamilies. Polyphyly of the Chioninae is expected to limit the extent to which chionine networks can be united.