Convergence, parallelism, and function of extreme parietal callus in diverse groups of Cenozoic Gastropoda

We use SEM 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. The extreme parietal callus 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 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 EPC -- uses less organic scaffolding and is less energetically expensive than normal shel...我们采用扫描电子显微镜（Scanning Electron Microscopy，SEM）成像技术，对5个新进腹足类（caenogastropods）科——凤螺科（Strombidae）、涡螺科（Volutidae）、橄榄螺科（Olividae）、拟橄榄螺科（Pseudolividae）以及安氏螺科（Ancillariidae）——的化石与现生物种的壳微观结构开展观测，旨在明确究竟是平行演化还是趋同演化造就了新进腹足类的一项独特特征：极端壁周胼胝体（extreme parietal callus，EPC）。极端壁周胼胝体被定义为螺层胼胝体与腹面壳表胼胝体的显著增厚，其覆盖面积可达壳表的50%及以上。整体而言，新进腹足类通过多种低密度、低有序度的微观结构趋同演化形成EPC，而这类微观结构在新进腹足类非胼胝体壳层的构建中较为罕见。然而在演化支内部，我们观察到壳层与胼胝体微观结构的调控程度均有所降低，这为平行演化提供了佐证。用于构建EPC的低密度、低有序度微观结构，相较于常规壳层结构，所需的有机支架更少，能量消耗也更低。