Size: all it’s shaped up to be? Evolution of shape through the lifespan of the Cenozoic bivalve <i>Spissatella</i> (Crassatellidae)

Abstract

Abstract Outline shape is a key morphological character of infaunal bivalves that reflects both phylogenetic history and function. A new method of Fourier shape analysis has been used here to study the evolution of shape and size in the Cenozoic genus Spissatella (Crassatellidae), using material from New Zealand. The Fourier method can be used to construct a map of morphological ‘geography’, to generate synthetic average or extreme morphologies, and to visualize marginal growth fields and allometries. The results demonstrate that growth in Spissatella was strongly allometric and that differences between individual growth stages are commonly far greater than evolutionary changes in shape spanning 20 Ma. In all taxa, allometry was dominated by the relative elongation of the posterior margin with growth, an inferred functional adaptation for life in relatively high-energy environments. Heterochrony was the dominant evolutionary mechanism and, following an initial peramorphic expansion into morphospace, both paedomorphosis and peramorphosis probably occurred in approximately equal proportions. In general, it is not possible to decouple patterns of shape and size evolution. The data reveal no directional trends in evolution of these traits. The ontogenetic pathway followed by Spissatella may be a key genus-level taxonomic character and apparently represents a developmental constraint that largely controlled evolution within the genus.