The shell microstructure,mineralogy and isotopic composition of Amussiopecten baranensis (Pectinidae, Bivalvia) from the Miocene of Spain: A valuable paleoenvironmental tool

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Amussiopecten baranensis ( Almera & Bofill, 1896 ), of the Family Pectinidae, is widespread in Miocenemarine sediments from northeastern and southeastern Spain, ranging in age from Early Langhian to Middle Messinian. The shell consists of an outer homogeneous and foliated layer composed of low-magnesium calcite, a middle layer aragonitic, and an inner foliated layer of low-magnesium calcite. Both left and right valves have, in general, the same microstructure and mineralogy except for an outermost thin polygonal layer in the earliest-formed dissoconch of the right valve. The homogeneous structure (dense and granular sublaminae) up to 50 μm thick overlies the sublayer of foliated calcite (regular, irregular and crossed foliated) that extends throughout the whole shell, being thinnest in the dorsal margin and thickening toward the ventral margin. The regular foliated inner layer extends from the beak, where it attains its maximum thickness, to near the adductor muscle scar. The middle layer is the most complex in structure and distribution, being composed of three sublayers which from exterior to interior are: 1) aragonitic simple crossed lamellar, 2) aragonitic prismatic (myostracal), and 3) aragonitic complex crossed lamellar. The latter sublayer in the internal shell surface extends to the area interior to the pallial line. Prominent features of the middle layer are the asymmetrical distribution of the prismatic structure (absent in the anterior margin), and the thinning of the three sublayers both in radial and antero-posterior directions. The divaricate microsculpture extending on the umbonal area and auricles in both valves originates from the granular calcite of the homogeneous structure. δ 180 values of the outer foliated layer from the ventral margin suggest that A. baranensis segregates its calcite in isotopic equilibrium with ambient sea-water, and thus affords a valuable tool in paleoenvironmental studies.