OBSERVATIONS ON CORBULID GROWTH AND THEIR EVOLUTIONARY SIGNIFICANCE

Corbulids are small, sturdy-shelled bivalved molluscs that range from the Late Jurassic through the recent. Their long geologic range notwithstanding, corbulid morphology remains relatively unchanged. In general, corbulids possess ovate to trigonal inequivalve shells: the right valve tending to be larger than the left. Surprisingly corbulids use various patterns of growth to achieve their adult shape. Recognition of these patterns may provide important environmental and evolutionary data.

Sclerochronologic and stable oxygen isotope (d¹⁸O) analysis of several groups illustrate a variety of growth patterns. Bicorbula idonea exemplifies isomorphic growth. d¹⁸O profiles reveal that individuals can live for nearly a decade. Growth lines show that in cross section the shape of juveniles is nearly identical to the adult. Together these data indicate that, despite being relatively long lived, they do not change shape through ontogeny. In contrast, other corbulids display allomorphic growth. Caryocorbula amethystina displays two distinct growth phases during its ontogeny. In its first year, the clam achieves its maximum dorsoventral shell height. Growth in following years thickens the entire shell while adding little to its overall height. This thickening phase can last at least three years. Corbula speciosa also demonstrates allomorphic growth, but the pattern is very different than Caryocorbula amethystina. The nepioconch is deposited predominantly during the first year of growth. Subsequent growth follows a dramatic morphologic discontinuity, marked by changes in growth direction, shape, and sculpture. This latter phase can last at least four years.

These examples reveal that corbulids employ a variety of growth patterns. Identification of growth strategies among different species is an integral part of ongoing studies of their evolution. Recent work indicates developmental constraint may play a role in restricting the morphology of some corbulids. Understanding how different species achieve their constrained shape may help untangle the importance of developmental versus environmental constraints. Similarly, if discrete patterns of growth are identifiable they can be used as additional character states to further refine phylogenetic reconstructions.