Position of soft parts in fossil palaeotaxodont bivalves as suggested by features of the shell interior

Markings on the shell interior of fossil palaeotaxodonts may indicate the position of the gonad and the gills, while umbonal muscle scars probably mark the attachment points of the muscular floor of the visceral cavity. A comparison of Ordovician to Recent nuculoids indicates a progressive change in the pattern of the umbonal scars, which may reflect the evolutionary development of the muscular visceral floor. Although the present form of the visceral floor does not seem to have developed in the nuculoids until the Triassic, it was well established in the nuculanids by the Devonian.