The evolution of functional design in rudists (Hippuritacea) and its taxonomic implications

Abstract

Poor understanding of rudist growth geometry and anatomy has hampered systematic studies of the superfamily. A flexible model that simulates the growth of rudist shells is therefore presented so that evolutionary trends in the group may be consistently analysed; this model is constructed by rotational or irrotational stacking of inclined gnomons around a contained axis. Functional analysis of shell geometry and reconstructed anatomy provides a more solid foundation for rudist systematics. The first rudists (Diceratidae) employed one or other of the spirogyrate umbones, inherited from megalodontid ancestors, as a facultatively elevating encrustation stem. Invagination of the ligament in the Caprotinidae permitted uncoiling of the shell, though this also entailed reduced gaping and therefore externalization of food entrapment, with increasing involvement of the mantle margins. Caprotinid functional design was preadapted to several new adaptive zones, which were exploited by various advanced descendant groups. Some of these groups show homeomorphic evolution and have often been assembled by earlier workers into polyphyletic ‘families’ (e.g. Caprinidae). An attempt is therefore made to establish a skeletal classification of rudists on the basis of true clades, as distinguished by careful functional analysis.