Article: The structural evolution of the bivalve shell

Direct study of the course of evolution of bivalve shell structures has been prevented by the lack of well-preserved lower Palaeozoic material. The 'primitive' molluscan shell structure probably consisted of an outer aragonitic prismatic layer, the prisms being polygonal in transverse and columnar in longitudinal sections. The middle and inner shell layers consisted of nacreous structures. Morphologically similar structures are produced inorganically from the solidification of metals containing impurities. It is suggested that the prism/nacre combination originally arose spontaneously as a result of the precipitation of calcium carbonate with protein (impurity). The subsequent elaboration of the shell structure combinations took place along seven major morphological trends. The main structural changes have been: the modification and loss of the outer prismatic layer; the elaboration of the middle layer from nacre into various other types of dendritic growth such as calcitic foliated or aragonitic crossed-lamellar structures; and the loss of organized structure to produce a homogeneous granular structure. In all the series there has been a progressive loss of layers from the 'primitive' three to a more 'advanced' two or even one.