Gastropod evolution and morphological diversity in relation to shell geometry

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Abstract
An analysis of shell form in conispirally coiled gastropods on the basis of three parameters developed by Raup (1966) reveals differences in morphological diversity in the several major taxonomic categories. Archaeogastropods, most “lower” mesogastropods, and many stylommatophoran pulmonates are characterized by transversely somewhat elongate to circular or ovate generating curves and a large angle E of elevation of the coiling axis. In the “higher” mesogastropods, neogastropods, basommatophoran pulmonates, some “lower” mesogastropods and many stylommatophoran pulmonates, the angle E is generally low, and there is a great diversity of form of the generating curve. A morphological argument is proposed to explain the observation that a large angle E is always associated with simple generating curves that are never longitudinally elongate, and that anterior inhalant siphonal openings and longitudinally elongate generating curves are invariably associated with low E values. It is suggested that a lowering of the axis of coiling from its primitive steep inclination has made it possible, in a geometrical sense, for gastropods to exploit more of the potential diversity inherent in their conispirally coiled permanent exoskeletons.

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