MODULARITY, INTEGRATION AND DISPARITY IN AN ACCRETIONARY SKELETON: ANALYSIS OF VENERID BIVALVIA

Modular evolution, the relatively independent evolution of body parts, may promote high morphological disparity in a clade, whereas integrated evolution via the tighter covariation of parts may limit disparity. But integration can also promote high disparity by channeling morphological evolution along lines of least resistance—a process that may be important in the accumulation of disparity among organisms with accretionary growth, as in many invertebrate systems. We use a time-calibrated phylogenetic hypothesis and high-density, 3D semilandmarking to analyze the relationship between modularity, integration, and disparity in the most diverse extant bivalve family: the Veneridae. Most venerids have a two-module parcellation of body features, but the rock-borers show complete modularity of the seven body features measured here. The more common two-module pattern is divided into features of the shell and features of the internal soft anatomy, suggesting a division falling more along developmental than functional lines when placed in the context of bivalve evolutionary-development and biomechanics. The strength of modularity varies by life habit, as does the integration of traits within modules, although disparity does not vary systematically with either. Instead, this clade shows multiple pathways to high disparity: high net diversification with anisotropic morphological evolution, and low net diversification with more evenly but widely distributed, isotropic evolution. Extending this approach to other bivalve families and other invertebrates with accretionary growth may corroborate such alternate pathways to broad disparity not tied to increasing modularity or weaker integration, a more common theme in systems with articulated skeletons.