Aspects of the adaptive morphology and evolution of the Trigoniidae

Abstract

During the Jurassic and Early Cretaceous, the Trigoniidae were the dominant family of shallow-burrowing bivalves of warm, shallow seas. Neotrigonia is the only genus that survives today. Judging from the biology of Neotrigonia and the functional morphology and occurrence of extinct genera, the Trigoniidae represent an advanced family of burrowing Bivalvia. The family has been characterized by efficient locomotion, owing in large part to the presence of an unusually muscular foot, resembling that of the living Cardiidae. Other unusual features of the Trigoniidae relate to the presence of the foot. Complex hinge teeth with secondary dentition evolved to maintain valve alignment at the wide angles of gape required for extrusion of the foot. Myophorous buttresses evolved to support the large anterior hinge teeth. These anterior features seem to have obstructed the evolution of a prosogyrous shape of the kind that facilitates burrowing within many other bivalve taxa. Alternatively, there evolved in the Trigoniidae various kinds of external shell ornamentation that aided in burrowing. Thus, the many unusual morphological features of the Trigoniidae have been strongly coadaptive. The Mesozoic Trigoniidae seem to have been more advanced animals than any shallow-burrowing, suspension-feeding bivalves of the Palaeozoic and also than certain successful living groups with similar modes of life. Though slightly less advanced than the Cardiidae, they can be regarded as the cockles of the Mesozoic. Had the group not suddenly been decimated by environmental deterioration at the end of the Mesozoic, it would undoubtedly flourish today. Neotrigonia has, in fact, speciated at a high rate in comparison with other living genera of bivalves. A relatively recent origin and lack of primitive features disqualify Neotrigonia from status as a living fossil genus. It is the sole survivor of its family, but an advanced and modern animal nonetheless.