The late Albian (Early Cretaceous) lineage of Actinoceramus sulcatus (Parkinson 1819) is a remarkable example of macro- and microevolution within the Bivalvia. Immediately following cladogenesis from ancestral A. concentricus, the lineage displays a conspicuous, short-term excursion through morphospace followed by a return to the ancestral form. This excursion is marked by the acquisition, and subsequent loss, of large radial folds affecting some part or all of the shell. The pattern is noteworthy because of the gross scale and rate of morphological evolution, the relatively short life span of an “extreme” morphology, an apparent evolutionary reversal, the presence of phenotypic clines through time, the extent of phenotypic variation within populations and abundance of morphological intermediates between disparate end-member types, the wide geographic distributions of phenotypic clines and variants, and the subtle asymmetry of morphological transitions bounding the evolutionary excursion.
From census and biometric analyses of stratigraphically constrained samples, we conclude that morphological change was not focused at speciation and the pattern of evolution does not conform to the classical paradigm of punctuated equilibrium. Instead, we infer that observed patterns are best explained by phyletic evolution, at widely varying rates, combined with ecophenotypic plasticity. Evolution targeted the potential to form radial folds; the expression of those in any individual was determined, in part at least, by environmental cues. Ecophenotypic plasticity in A. sulcatus was itself probably an evolutionary response favored by the presence of long-lived planktotrophic larvae and wide dispersal of the species. In A. sulcatus, there is a continuum of pattern between intrapopulational, ecophenotypic variation that can be observed on bedding planes, interregional variation, and phyletic change through time. We argue that this continuity of pattern is most easily explained by continuity of process: in the case of the visually striking radial folds in A. sulcatus, there is no reason to invoke distinct hierarchies of macro- and microevolution; instead, these seem to be parts of a continuum.