Shell microstructure and mineralogy of the mollusc species Anadara uropigimelana (Bory de Saint-Vincent, 1827), Tivela stefaninii (Nardini, 1933) and Oliva bulbosa (Röding, 1798)

Mollusc shells are composite structures build of crystals of calcium carbonate (calcite, aragonite, or both) and biopolymers, arranged in a great variety of microstructures. Shell formation is affected by environmental and physiological factors and differences in shell microstructures are believed to be of phylogenetic and adaptive biomechanical significance. Here, we performed a detailed characterization and illustration, through SEM and XRD analyses, of the shell microstructure and mineralogy of specimens belonging to the bivalves Anadara uropigimelana (Bory de Saint-Vincent, 1827) and Tivela stefaninii (Nardini, 1933), and to the gastropod Oliva bulbosa (Röding, 1798), collected in the Upper Holocene (Meghalayan) HAS1 settlement and in a shell midden in the Khor Rori Archaeological Park (Dhofar, Oman). Specimens of Anadara uropigimelana show an aragonitic shell with an outer crossed lamellar layer, an inner complex crossed lamellar layer and an irregular simple prismatic pallial myostracum; furthermore, periodic bands of dendritic nondenticular composite prisms occur in the outer part of the outer layer, reflecting seasonal changes in water temperatures. Specimens of Tivela stefaninii show an aragonitic shell with an outer composite prismatic layer, a middle crossed lamellar layer and an inner complex crossed lamellar layer, whereas specimens of Oliva bulbosa have an aragonitic shell characterised by an irregular alternation of crossed lamellar layers. With this investigation we aim to provide novel data on the shell fabric of these poorly known mollusc species, increasing the availability of useful characters for phylogenetic, evolutionary, palaeoenvironmental and crystallographic studies.