The balanced life: evolution of ventral shell weighting in gastropods

Abstract

Abstract Exaggerated morphologies present challenges for understanding the factors that enable, prevent or compel lineages to evolve unusual functional traits. Here, I examine the case of gastropods whose adult shells are bottom heavy by virtue of a thick, often broad, convex ventral callus deposit on the inner lip and adapical to the aperture. Data on the spatial, temporal and phylogenetic distribution of callus-bearing gastropods were gleaned from the literature and from personal observations on fossil and living gastropods in collections. An exaggerated ventral callus has evolved in ≥ 72 lineages of gastropods, all in warm, shallow waters, beginning in the Late Ordovician but mostly during and after the Late Cretaceous. The phylogenetic distribution of these gastropods is highly clumped, with multiple acquisitions in some clades (e.g. Stromboidea and Buccinoidea) and none in others (e.g. Conoidea). Functional considerations and comparisons indicate that the enhanced gravitational stability provided by a ventral callus evolved only in conditions of low-cost calcification and when predators capable of dislodging or overturning gastropods or causing them to flee were important agents of selection.