Article: Functional morphology, ecology, and evolutionary conservatism in the Glycymerididae (Bivalvia)

Since its appearance near the beginning of the Cretaceous, the family Glycymerididae has retained the same simple shell form. Variation among species is largely restricted to differences in size and external sculpture. This evolutionary conservatism can be explained in terms of the morphology and ecology of Glycymeris. Bivariate and multivariate studies of interactions among shell characters show that individual parameters of the shell are closely interrelated; there are rigid geometrical and mechanical constraints on deviations from the simple form. Relative growth of most characters is not strongly allometric; where marked allometry does occur, notably in the growth of the ligament, it is directly related to the over-all size of the animal. Thus the potential for evolutionary change in shell form by heterochrony is limited. Glycymerid soft-part anatomy is unspecialized and the animal is functionally less efficient in several respects than more advanced bivalves. Glycymerids have apparently always occupied the same current-swept marine environments. They evolved as functional generalists, adapted to a physically rigorous environment. The compromises that were essential to this adaptation left the group with insufficient flexibility of form to radiate into a wide variety of environments, whence its conservatism.