Ontogenetic variation and inoceramid morphology: a note on early Coniacian Cremnoceramus bicorrugatus (Cretaceous Bivalvia)

Abstract

Taxonomic studies of Cretaceous inoceramid bivalves are confounded by high levels of intraspecific morphological variation within the group. Such variation is illustrated here using Cremnoceramus bicorrugatus s. s. (MARWICK, 1926) from the lower Coniacian of New Zealand. This taxon displays a remarkable range of intraspecific variation that is the sum of three major components: 1) a basic set of ontogenetic transformations between the juvenile, immature and adult stages; 2) intra-populational variation in the relative timing or rate of developmental events; and 3) intra-populational variation in the rate of absolute growth. The first of these components was largely genetically determined, whereas the other two were probably influenced by extrinsic environmental factors. Taxonomic interpretation of C. bicorrugatus s. s. is not possible without some understanding of both the basic ontogenetic plan and developmental variations within that plan. Interpretation is facilitated by the profound and step-wise nature of the ontogenetic transformations. In many other inoceramids, however, ontogenetic transformations are more subtle but may be subject to equally significant intraspecific developmental variations. Therefore, inoceramid taxonomists should describe not only total population variation, but also should document ontogenetic development in any species. In this way it is possible to identify basic ontogenetic plans, constrain likely limits to intraspecific variation, and distinguish intraspecific from interspecific morphological variation.