EVALUATING ECOLOGICAL FUNCTION AND CONVERGENT EVOLUTION OF EXTREME PARIETAL CALLUS IN MARINE GASTROPODS

We used the development of the “extreme parietal callus” (EPC), defined here as callus which covers more than 50% of the ventral shell surface, to investigate gastropod homoplasy by parallel and convergent evolution while testing possible ecological functions of this feature. Examination of EPC macro and microstructure suggests convergent evolution within and between four marine gastropod families: Strombidae, Olividae, Pseudolividae, and Volutidae.

The shell and EPC of the Miocene strombid fossil Orthaulax is composed of crossed lamellar aragonite, which is interpreted as a well-constrained, energy-intensive depositional processes. The shell, callus, and EPC in the Recent olivids Olivancillaria and Amalda are also crossed lamellar. Braiding of complex crossed lamellae binds discrete shell horizons in Olivancillaria while a prismatic layer separates shell from callus deposition in both genera. In the Eocene fossil olivid Ancillopsis altilis, the EPC iss built from weakly-ordered fibrous, irregularly-ordered prismatic, and homogenous growth with interspersed prismatic lamina. We interpret the poorly-ordered microstructure of A. altilis as a low-energy alternative converging on highly-ordered EPC construction exhibited by other olivid genera. In fossil Pseudolividae, normal callus and EPC possess weakly-ordered fibrous prismatic microstructure with occasional prismatic lamina while EPC has regions of irregularly-ordered prismatic growth, similar to the structure and implied energy requirements of A. altilis. In the Volutidae, the spines and callus of Eocene Athleta rarespina are composed of more energy-intensive crossed lamellar microstructure, while A. symmetricus spines are constructed from weakly-ordered fibers. In A. tuomeyi, the microstructure of EPC is poorly-ordered and massively constructed, representing yet another convergent approach to producing EPC which does not derive directly from the well-ordered callus deposition in other Athleta.

In wave tank experiments, Athleta tuomeyi with EPC showed reduced transport and tumbling by wave activity compared to A. symmetricus without callus. Corroborating previous work on the sedimentology and distribution of fossil and recent specimens, we suggest EPC might function as ballast in relatively high-energy environments.