An ontogenetic perspective on the evolution of shell size and shell shape in the land snail genus Plectostoma
The rampant convergent and parallel evolution in shell form in the Gastropoda is well known. Many studies focus on the functional drivers which have been regarded as a major force in shell evolution. There is, however, a scarcity in studies that aim at understanding shell form evolution with respect to their ontogeny. Hence, we investigated the evolution of shell form in the micro-landsnail genus Plectostoma (Diplommatinidae) from the viewpoint of shell ontogeny. We examined the aperture ontogeny profiles that describe how aperture form and growth trajectory change along the shell ontogeny, and how the aperture ontogeny profiles relate to the observed shell forms. We also estimated the phylogeny of Plectostoma species, and examined patterns of character evolution for shell form. Our study revealed a general issue in the characterisation of shell shape and demonstrated how shell shape differences can be expressed as differences in the ontogeny of morphospace. It is clear that in Plectostoma the phylogenetic history does not prevent the course of shell ontogeny, and the resultant form. Finally, each species has a unique aperture ontogeny profile that determines its shell shape while retaining a conserved developmental program that maintains shell size.