MICROSTRUCTURE ANALYSIS OF UNUSUALLY EXTREME PARIETAL CALLUS IN FOSSIL NEOGASTROPODS

Homoplasy is generally thought to be widespread in gastropod shells, but can be difficult to study, particularly in fossils. Microstructure, however, may reveal additional characters that can be used for such analysis. The callus, a secondary shell layer on the inner or outer lip, is a common feature in gastropod shells, but its structure and function are poorly understood. Extreme parietal callus (EPC) – when the parietal callus is extended over more than half of the ventral surface and any of the spire -- occurs across a surprising array of marine gastropods, including at least 8 families (especially Olividae) and more than 30 genera, Paleocene-Recent. It is a homoplasious structure based on its phylogenetic distribution. We examined EPC in three species from the Lower Eocene of the U.S. Gulf Coastal Plain, and found it to be constructed by the same shell layer. However, it is formed in two macrostructural “modes” and three different patterns of microstructure. Ancillopsis [commonly, but erroneously, referred to as Bullia] altilis (Conrad) (Olividae) forms the callus in short bands in pseudo-crossed lamellar structure. Sulcobuccinum [commonly referred to as Pseudoliva] santander (Gardner) (Pseudolividae) also forms the callus in short bands, but in blocky prismatic structure. Athleta tuomeyi (Conrad) (Volutidae) forms it in long layers in irregular prismatic structure. When combined with simple morphogenetic models and Recent observations on the formation and possible functions of EPC, this study may provide useful insights into the relative roles of selection and constraint affecting gastropod shell form.