To achieve a better understanding of the formation and evolutionary history of molluscan shell microstructures, we analysed crystallographic textures of shell microstructures of selected species of Protobranchia, the most ancient group of Bivalvia. Our dataset covers four of the five protobranch superfamilies. Shell layers of five species of Nuculoidea, four species of Solemyida and seven species of Nuculanoidea were analysed and classified according to their textural crystallographic patterns into six different types. Our investigation revealed that some microstructures (e.g. the radially elongate simple prismatic structure) have structure-specific textural patterns, while other microstructures (e.g. the homogeneous structure) show taxon-specific differences despite morphological similarities. The former result means that each microstructure accurately reflects a unique crystallographic structure; the inconsistency in the latter, however, calls into question the homologies among morphologically defined microstructures. In comparison with the latest molecular phylogenetic trees, our data showed that crystallographic textures are phylogenetically constrained within Protobranchia. Each nuculoidean species had two to three distinct textural patterns, and one or two similar textural patterns were recognized within each species of Solemyida and Nuculanoidea. In general, closely related species had similar crystallographic textural patterns regardless of differences in shell microstructures.
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