Bivalve systematics across the fossil–Recent divide

The systematics of a clade is the foundation upon which any further study must be built. The molluscan class Bivalvia presents challenges to the systematist – homoplasy, deep splits in the tree and many species known only from shells – but equally presents opportunities. Steady progress has been made in many areas, and understanding of the clade’s phylogeny has stabilized in the last decade, to universal benefit. The excellent fossil record of bivalves means that patterns of past diversity and disparity can be measured, rather than estimated, enabling combined neontological and palaeontological approaches to further develop the Bivalvia as an unparalleled macroevolutionary study system. Recent advances in molecular sampling, the recovery of DNA from historical specimens, imaging and morphometrics have strengthened bivalve systematics, but relationships among molecular clades, morphological concepts and fossil lineages remain difficult to reconcile. Here, we discuss selected advances in bivalve systematics and phylogenetics, identify important conflicts and gaps between a synoptical classification and a family-level phylogeny, and propose future work centred on two linked problems: reconciling molecular and morphological evidence for relationships among living lineages, and integrating fossil lineages into phylogenetic hypotheses. We argue that such total-evidence phylogenies need not be assembled in a single step. Densely sampled, vouchered molecular backbones, quantitative tests of morphological characters against those backbones and fossil placement using morphology and stratigraphy can proceed in parallel, provided that each is designed to inform the others.