A new look at early bivalve phylogeny

Abstract

Abstract In the 30 years since publication of the bivalve Treatise , (Moore, R. C. (ed.) 1969. Treatise on Invertebrate Paleontology. Part N. Mollusca 6, Bivalvia , Geological Society of America and University of Kansas) important new faunas have been described from the early and mid Cambrian and from the early and mid Ordovician. These contain significant new forms, including some long-ranging intermediate groups, that indicate the relationships between the principal bivalve clades, but lack of fossils from the late Cambrian and earliest Ordovician is a major hindrance. The principal phase of bivalve diversification followed on from the evolution of the filibranch gill in the latest Cambrian or earliest Ordovician. The fundamental division of the class is into two subclasses, Protobranchia and Autolamelli branchiata; links between the two can be demonstrated in the early Ordovician. Major divisions of each subclass are recognized as superorders. Within the Protobranchia, the Nuculoida developed specialist food-gathering palps and an enlarged foot. Diverging early from the protobranch stock were other bivalves that lived symbiotically with sulphur-oxidizing chemoautotrophic bacteria; this allowed colonization of anaerobic substrates and produced two distinct stocks: the deeply infaunal anteriorly elongate Solemyoida and the shallower infaunal Nucinelloida. The Autolamellibranchiata, initially identified by strongly asymmetrical hinges, diversified in three directions, each characterized by distinctive hinges. The Trigonioida were characterized by ligamental nymphs and frequently denticulate teeth, and rapidly regained greater symmetry; the Anomalodesmata also developed a strong ligamental insertion within nymphs and largely lost their dentition, whilst the Heteroconchia, principally with a shell including a complex crossed-lamellar structure, had various combinations of cardinal and lateral teeth. Heteroconch diversifications were mainly in the Mesozoic and Cenozoic, but one Ordovician group, the Glyptarcoidea, is a good ancestor for the Pteriomorphia. The following new taxa are proposed: Cardiolarioidea superfam. nov., Eritropidae fam. nov. and Catamarcaidae fam. nov.