Molluscan marginalia: hidden morphological diversity at the bivalve shell edge

Molluscan shells exhibit a high and largely neglected diversity of serrations and crenulations at the growing margin. A survey of living and Cenozoic fossil bivalves indicates that serrations, in which the external ribs or interspaces between ribs extend radially beyond the general contour of the valve margin, may be symmetrical or asymmetrical. Projections whose adumbonal edge more nearly parallels the shell edge than the abumbonal edge occur on the posterior valve margins of many limids, cardiids and donacids, as well as in the arcid Anadara and a few concentrically ridged tellinids. In many cardiids, posterior serrations form as extensions of ribs, whereas ventral and anterior projections are extensions of rib interspaces. Asymmetrical serrations are almost always found on shell edges that are polished, indi-cating that the mantle extends slightly over the valve margin to the outside. I tentatively suggest that asymmetrical serrations enable the valves to close around siphons or other mantle extensions without in-juring these soft tissues, so that the bivalve can maintain sensory contact with the environment even while the shell is shut. A preliminary comparison with brachiopods indicates that the diversity of condi-tions at the shell edge is much higher in bivalves. Together with the largely warm-water and marine dis-tribution of marginal modifications in bivalves, these comparisons reflect the higher metabolic potentials of bivalves relative to brachiopods, and show that the shell edge is a rich source of evidence about function and mode of life in fossils