Evolutionary origins of novel conchologic growth patterns in tropical American corbulid bivalves.

We conducted a combined sclerochronologic and phylogenetic analysis to document patterns and rates of shell accretion in several subclades of related corbulids, and to explore the evolutionary origins of novel conchologic developmental patterns. We found three disparate patterns of valve development in Neogene tropical American corbulid genera. These patterns include growth through primarily radial accretion along the sagittal plane, and two derivative patterns: one characterized by initial deposition of a thin shell followed by valve thickening with little increase in valve height, and another producing a well-defined nepioconch through a marked change in the primary growth direction. We conducted a species-level phylogenetic analysis of the taxa surveyed for growth patterns, focusing on the ([Bothrocorbula+Hexacorbula]+Caryocorbula) clade. The phylogenetic distribution of shell growth patterns suggests that this clade is characterized by derivative patterns of growth. Oxygen-isotope calibrated ontogenetic age estimates of species in the derived Bothrocorbula subclade further suggest that transitions from the ancestral radial (sagittal) growth pattern to a derived pattern of growth are a function of heterochrony (peramorphosis by acceleration). These findings are significant because they link previously observed patterns of morphological constraint with a specific evolutionary process, demonstrate how morphologic constraint and innovation can be interrelated, and serve as a model for understanding the evolution of morphologic diversity in the clade as a whole. Furthermore, this study highlights the utility of sclerochronologic records as an important component of evolutionary developmental research on organisms with accretionary skeletal growth.