Investigating the shell microstructure and mineralogy of three Upper Holocene mollusc species from the Khor Rori Archaeological Park (Dhofar, Oman)

Mollusc shells are composite structures built of calcium carbonate crystals (calcite and/or aragonite) and biopolymers (protein, polysaccharides and lipids). These two classes of materials create a great variety of microstructures characterised by complex architectures and unique material properties. The formation of shell microstructures is controlled by environmental and physiological factors and the variety of microstructures is believed to be of phylogenetic and adaptive biomechanical significance. Here, through SEM and XRD analyses, we provide for the first time a detailed characterisation, description and illustration of the shell microstructure and mineralogy of three mollusc species: the bivalves Anadara uropigimelana (Bory de Saint-Vincent, 1827) and Tivela stefaninii (Nardini, 1933), and the gastropod Oliva bulbosa (Röding, 1798). The specimens were collected in the Upper Holocene HAS1 settlement and in a shell midden in the Khor Rori Archaeological Park (Oman). These species frequently occur in archaeological assemblages and show clear growth lines and increments in their shells, making them excellent tools to be used for high resolution palaeoclimatic and palaeoenvironmental studies. However, data on their shell microstructure and mineralogy, that are fundamental for fossil shell preservation analyses, are lacking. Our results show that shells of Anadara uropigimelana are aragonitic with an outer crossed lamellar layer, an inner complex crossed lamellar layer and an irregular simple prismatic pallial myostracum; periodic bands of dendritic nondenticular composite prisms occur in the outer part of the outer layer, reflecting seasonal changes in water temperatures and growth rates. Tivela stefaninii shells are aragonitic and show an outer composite prismatic layer, a middle crossed lamellar layer, and an inner complex crossed lamellar layer. Shells of Oliva bulbosa are composed of an irregular alternation of aragonitic crossed lamellar layers; a transitional layer characterised by the occurrence of tidally controlled growth lines, a crossed lamellar callus and a myostracal layer are also described in Oliva bulbosa specimens. With this analysis, we are able to provide novel microstructural and mineralogical data on three poorly known mollusc species, which are useful for crystallographic, phylogenetic, evolutionary and palaeoenvironmental studies.