Biometric analysis, systematics and evolution of Albian Actinoceramus (Cretaceous bivalvia, Inoceramidae)

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Crampton, J.S. 1996 Biometric analysis, systematics and evolution of Albian Actinoceramus (Cretaceous bivalvia, Inoceramidae). Lower Hutt: Institute of Geological & Nuclear Sciences. Institute of Geological & Nuclear Sciences monograph 15; New Zealand Geological Survey paleontological bulletin 71 74 p.

Abstract: Attempts to formulate a robust classification of the bivalve genus Actinoceramus (family Inoceramidae) have hitherto foundered because of widespread homoplasy in the group, high levels of intraspecific variation, and difficulties of objectively describing this variation. These problems are common to the family as a whole, and can only be addressed through detailed biometric and biostratigraphic studies. One biometric tool which is particularly suited to the study of such bivalves is elliptic Fourier shape analysis (EFA) of the shell outline. In this paper EFA is applied to the inoceramids for the first time, and is instrumental in clarifying the taxonomy and phylogeny of Albian Actinoceramus. Interpretations are based primarily on well preserved and, in many cases, topotypic material from well studied English sections. Inferences drawn from these collections are extended using material from Switzerland, South Africa, New Zealand, and Antarctica. Evolutionary transitions between taxa are gradualistic at the resolution of sampling, and successive forms generally overlap in morphospace. For this reason species, and probably the genus itself, cannot be uniquely defined using morphology alone, and diagnoses must take into account biostratigraphic or phylogenetic criteria. Three species of Albian Actinoceramus are recognised: A. salomoni, A. concentricus, A. sulcatus. Species boundaries are defined at apparent intervals of rapid evolution or inferred cladogenetic events. Distinct morphologies within species are given the status of subspecies. Three new subspecies are described. A. concentricus expandoclunis, A. concentricus parabolicus, and A. sulcatus biometricus. Existing data suggest a steady increase in the biogeographic range of the genus throughout the Albian. During the early and middle Albian, Actinoceramus was possibly restricted to the Tethys and proto-Indian Ocean; by the late Albian this range extended at least into the North American Seaway and North Pacific. (auth)

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