A BIT OF EVERYTHING, QUICKLY! PHENOTYPIC VARIATION IN EARLY CRETACEOUS ACTINOCERAMUS (BIVALVIA)

Collections of the cosmopolitan bivalve genus Actinoceramus from the lower upper Albian (Early Cretaceous) display a remarkable range of phenotypic variation. This variation is expressed within a single character suite, namely high amplitude, radial to antimarginal (i.e., margin-normal) folds in the shell (“radial elements”). Actinoceramus sulcatus s.s. is characterised by radial elements over the entire shell; ancestral A. concentricus and descendant A. sulcatus biometricus both lack radial elements. Between these end-member forms is a complete spectrum of morphologies. Biometric and census analyses of morphotypes suggest that, over a short period of about 1.1 Ma, the sulcatus lineage experienced rapid to gradual anagenesis together with rampant ecophenotypic plasticity. Phenotypic plasticity was expressed as the ontogenetic stage at which, and degree to which, radial elements formed in any individual. We infer that the first appearance of potential radial elements was an evolutionary event that exploited pre-existing morphogenetic machinery. This event was exactly coincident with significant, global oceanographic changes at the beginning of the late Albian, changes marked by pronounced warming, transgression, productivity changes and, possibly, Oceanic Anoxic Subevent 1c. In general, radial elements probably conferred a functional and selective advantage that may have been linked in some way to these oceanographic changes. Formation of radial elements, however, must also have imposed a physiological cost to the individual. Thus, under certain, local environmental or ecological conditions, “cost-benefit analysis” did not favour the formation of radial elements that were consequently retarded, reduced, or suppressed altogether. In other words, the potential to produce radial elements was an evolutionary innovation, the expression of those elements in any individual was determined by environmental cues. Phenotypic plasticity was itself probably an evolutionary response favoured by the presence of long-lived and widely dispersed planktotrophic larvae: plastic responses to environmental cues maximised phenotypic adaptability in the population whilst minimising physiological costs to the individual.