ROLE OF CONSTRAINT AND SELECTION IN THE MORPHOLOGIC EVOLUTION OF CARYOCORBULA
(MOLLUSCA: CORBULIDAE) FROM THE CARIBBEAN NEOGENE

We examined patterns of morphologic evolution in Caryocorbula, a common bivalve genus of the Caribbean Neogene, to evaluate the roles that constraint and selection play in its apparent morphologic conservatism. With multivariate analyses using landmark-based geometric morphometrics, we examined the relationship of valve size and outline shape within the Caryocorbula + (Bothrocorbula + Hexacorbula) clade, and within 21 morphospecies of Caryocorbula, all from Neogene deposits of the Caribbean region. We found that each genus is morphologically distinct both in shape and size, and that Caryocorbula shows strong interspecific allometry that persists despite species turnover and the vagaries of geographic and temporal sample coverage.

Persistence of this allometric trend indicates that constraints shape Caryocorbula morphology. Constraint mechanisms may relate to 1) space limitations in the inequivalved shells that characterize Corbulidae, and/or 2) the need for functional articulation of unequal valves through a complex shell accretion history. Alternatively, environmental conditions, particularly nutrient availability, may control the spatial and temporal distribution of large Caryocorbula, but constraint via pleiotropy could restrict the shape of these bivalves. Finally, although constraints appear to play an important role in the morphologic evolution of Neogene Caryocorbula, an adaptive relationship of size and shape to environment is not precluded, as illustrated for Caryocorbula species from the late Miocene-early Pliocene of the northern Dominican Republic. The morphology of these species falls along the interspecific allometric curve, but each is also characteristic of a particular paleoenvironmental setting, with valve size and shape tracking with inferred depth and physical energy gradients.