SEPTA AREN’T RARE IN GASTROPODS!

While septa are observed in many Paleozoic gastropod groups, they are often considered rare in modern gastropods. Notable exceptions to this statement in the literature include Turritellidae, Vermetidae, and decollate terrestrial snails. (Septa of Turritella communis were even compared with those found in Nautilus in Lyell’s Elements of Geology (1838).) Septation is, however, not rare within these groups and in fact can be found in numerous other Caenogastropod families including Cerithiidae, Muricidae, Batillariidae, Ranellidae, Caecidae and Campanilidae. This wide phylogenetic distribution of septa suggests that septa are either: 1) plesiomorphic for gastropods, or 2) frequently independently evolved, either convergently or through parallelism. Examination of septal microstructure is a first step in understanding how these structures evolved.

We have examined the composition, microstructure, and patterns of septal insertion in 11 turritellid species, both fossil and Recent. Septa were found to be very thin (0.14-20 mm), with thicker septa (up to 0.80 mm) composed of multiple thin septa fused together. Number and spacing of septa was found to be highly variable among species. Septa also appear to be generated in continuity with secondary thickening of the shell wall in many species. Septa within both Turritellidae (Cerithioida) and Ranellidae (Littorinimorpha) (Bandel, 1990) appear to exhibit crossed lamellar microstructure, indicating significant energy investment by the animal. Functional significance of gastropod septa remain unclear, but possibilities include prophylaxis against apical breakage, defense against predation, response to excess calcium, or ballast. In any case, septa are not rare in gastropods.