Article: Coadaptation in the Trigoniidae, a remarkable family of burrowing bivalves

During the Mesozoic Era, the Trigoniidae became the dominant family of shallow-burrowing bivalves of near-shore marine habitats. Several bizarre morphological features make this the most unusual large family of burrowing bivalves ever to have existed. Forming a coadapted complex, these features enabled Mesozoic trigoniids to burrow with great efficiency, which largely accounts for their evolutionary success. Their adaptive zone included numerous niches in habitats characterized by coarse, shifting substrata. Enormous complex hinge teeth evolved to keep trigoniid valves aligned at wide angles of gape required by a large, muscular, cockle-like foot. The dentition and, by inference, the enormous foot evolved during the Triassic and ushered in a remarkable adaptive radiation. In many species, unusual arrays of knobs or ridges on the shell surface gripped the sediment to facilitate burrowing.Trigoniids resemble cardiids and can be regarded as the cockles of the Mesozoic, but had less mobility, in part because of the friction of their complex hinge teeth. None the less, Mesozoic trigoniids were more advanced bivalves than are the Anadarinae, which have radiated in the Cenozoic. Had the trigoniids not suffered almost total extinction at the end of the Cretaceous, they would remain diverse today.