Article: The molluscan periostracum: an important constraint in bivalve evolution

The temporary, and often inconspicuous, nature of the bivalve periostracum has led to its relative neglect. As the first part of the valve to be secreted, onto which the calcareous shell is then deposited, the periostracum has great potential in governing basic valve form. Herein, periostracal thickness is shown to be an extremely variable character amongst members of the Bivalvia but within specific clades and life habits the range of variation is much narrower. It is suggested that differences in periostracal thickness have played a vital role in the evolution of different bivalve clades acting as an important preadaptation and constraint in the formation of various valve morphologies. Fine flexible periostraca allow the manufacture of intricate shell morphology and ornamentation, whilst a thickened sheet protects the calcareous part of the shell against environmental and biotic agents of dissolution.Despite the poor preservation potential of the molluscan periostracum, its evolution and its primitive state are worthy of consideration. The distribution of periostracal thickness in Recent bivalves and the relative lack of fine ornament in early members of the class suggest that it was of at least moderate thickness. The findings of this survey have potential relevance to other molluscan classes and other phyla which use an analogous method of shell construction.