Crenulations of the hinge plate of pectinoideans

Checa, A.G., Esteban‐Delgado, F.J. and Salas, C. 2010. Crenulations of the hinge plate of pectinoideans. — Acta Zoologica (Stockholm) 92 : 344–354. Crenulations are a succession of micron‐sized elongated ridges and troughs with a general dorsoventral pseudo‐labyrinthine pattern that are present on the hinge plate of most pectinoideans. The crenulations of opposing valves are complementary and interpenetrate in a hinge‐like fashion. They are also imprinted in the dorsal mantle, although there is evidence that the mantle adheres only at the ridges, but not at the troughs. Those mantle areas that secrete the ridges contain cells that are richer in structures, indicating an active metabolism than those facing the troughs. Crenulations are formed because the dorsal mantle expands allometrically along the anteroposterior direction. The excess length is accommodated by creasing. Crenulations initiate at the anterior and posterior ends of the hinge plate as shallow spheruliths, which increase in height with time until the mantle on both sides detaches from the shell areas forming the neighbouring troughs. A contact differentiation of the mantle into actively and passively secreting bands follows. This genetic model explains the general pattern as well as the changes in the morphology of crenulations that occur during growth. Crenulations are secondary postlarval structures unrelated to the provinculum. Their presence in the earliest pectinoideans makes crenulations a plesiomorphic character of the group.