Article: The function of zigzag deflexions in the commissures of fossil brachiopods

The commissures and valve-edges of many fossil brachiopods are deflected into a zigzag form. During ontogeny, this often produced radial ribs (costae) on the shell. Schmidt's hypothesis, that zigzag valve-edges acted as protective devices by transforming the apertures into narrow zigzag slits, is extended by determining the paradigm for this function. The zigzag deflexions of fossil brachiopods approximate, often very closely, to this paradigm; they are therefore interpreted as protective devices. The nature of the 'protection' conferred is discussed in the light of observations on the physiology of living brachiopods. The gradual development of paradigmatic zigzags during the later stages of ontogeny is interpreted in terms of the increasing degree of protection required as the animal grew in size. The presence of species with zigzag deflexions does not imply any special environmental conditions. The rigid specification of the paradigm, and the existence of closely similar zigzags in fossil and living oysters, suggest the intrinsic probability that zigzags were evolved many times independently. This is confirmed extrinsically by the varied origin and development of zigzags during ontogeny, and by their scattered distribution in geological time and among the major taxonomic groups. Examples are cited from most superfamilies of the Articulata, and from horizons ranging from Middle Ordovician to Eocene.