Overlooked shell features: asymmetrical columellar folds in volutid gastropods

Abstract

Many gastropods have columellar folds, defined as continuous spiral ridges extending from the aperture to the shell interior on the adaxial side of the aperture. Despite their ubiquity, columellar folds have received little phylogenetic or functional study. In the vast majority of cases, columellar folds have a symmetrical profile, with the anterior (abapical) slope similar to the posterior (adapical) slope. Here I draw attention to the highly unusual phenomenon of asymmetry of columellar folds, where the anterior slope is markedly less inclined than the posterior slope of each fold. I have detected this condition in the volutid genera Scaphella Swainson, 1832; Cymbiola Swainson, 1831 and Lyrischapa Aldrich, 1911; as well as in the Palaeocene and Eocene genus Eovasum Douvillé, 1920. Following a review of Lyrischapa and Eovasum, I suggest that these two genera are closely related members of the volutid subfamily Indovolutinae, and that asymmetrical columellar folds have evolved three times in Volutidae. Although their function remains elusive, asymmetrical folds might prevent abapical displacement of the soft parts during rapid extension of the foot or feeding organs.