Homoplasy at Work: Convergent Formation of Extreme Parietal Callus In Multiple Lineages of Marine Gastropods

Homoplasy is widespread in gastropods, but its causes remain unclear; both natural selection and constraints are involved, but their relative contributions are difficult to identify. The parietal callus is a layer of secondary shell on the inner lip of the aperture. Its formation is not well understood, but it is clearly laid down by the mantle borne by the foot when it is extended from the aperture. In most snails, the callus extends barely out of the aperture. In others, it extends over the entire ventral, or even part or most of the dorsal, portion of the body whorl, and even over part or all of the spire. We call the extension of the parietal callus over more than half of the ventral surface and any of the spire "extreme parietal callus"(EPC).

EPC occurs across a surprising array of marine gastropods, including at least 8 families (especially Olividae) and more than 30 genera, Paleocene-Recent. We have explored EPC in detail in 3 species from the Lower Eocene of the Gulf Coastal Plain: Ancillopsis [commonly referred to as Bullia] altilis (Conrad) (Olividae); Sulcobuccinum [commonly referred to as Pseudoliva] santander (Gardner) (Pseudolividae); and Athleta tuomeyi (Conrad) (Volutidae). Phylogenetic analysis shows that EPC is homoplasious in these 3 species. Preliminary analysis of serial sections through the spires of S. santander and A. tuomeyi suggests that they formed their EPC differently: S. santander deposited it simultaneously with apertural growth, whereas A. tuomeyi formed it later. (Analysis of A. altilis is underway.) These two modes of EPC formation may have resulted from constraints caused by different coiling parameters in the two species. When combined with models and Recent observations of possible function of EPC, this study may provide useful insights into the relative roles of selection and constraint in gastropod shell form.