AbstractWe use scanning electron microscopy imaging to examine the shell microstructure of fossil and living species in five families of caenogastropods (Strombidae, Volutidae, Olividae, Pseudolividae, and Ancillariidae) to determine whether parallel or convergent evolution is responsible for the development of a unique caenogastropod trait, the extreme parietal callus (EPC). The EPC is defined as a substantial thickening of both the spire callus and the callus on the ventral shell surface such that it covers 50% or more of the surface. Caenogastropods as a whole construct the EPC convergently, using a variety of low-density, poorly organized microstructures that are otherwise uncommon in caenogastropod non-callus shell construction. Within clades, however, we see evidence for parallelism in decreased regulation in both the shell and callus microstructure. Low-density and poorly ordered microstructure—such as used for the EPC—uses less organic scaffolding and is less energetically expensive than normal shell microstructure. This suggests the EPC functions to rapidly and inexpensively increase shell thickness and overall body size. Tests of functional ecology suggest that the EPC might function both to defend against crushing predation through increased body size and dissipation of forces while aiding in shell orientation of highly mobile gastropods. These interpretations hinge on the current phylogenetic placement of caenogastropod families, emphasizing the essential contribution of phylogeny when interpreting homoplasy.