A multilocus phylogeny of archiheterodont bivalves (Mollusca,Bivalvia, Archiheterodonta)

The bivalve clade Heterodonta encompasses more than half of the extant bivalve species and is presently considered a derived group of the modern bivalves (Newell 1965 ; Waller 1998 ). Heterodonta is subdivided into two major lineages, the hyperdiverse Euheterodonta and Archiheterodonta. The latter comprises four relatively small extant families: Astartidae, Carditidae, Condylocardiidae and Crassatellidae, whose relationships and internal phylogeny are poorly understood. We assessed the phylogeny of archiheterodont bivalves using a multilocus data set comprised of molecular sequence data from six loci (18S rRNA, 28S rRNA, cytochrome c oxidase subunit I, cytochrome b, internal transcribed spacer 2 and histone H3). Resultant data sets of ~4 Kb of concatenated molecular sequence data were analysed using probabilistic approaches (maximum likelihood and Bayesian inference) and parsimony direct optimization. We recovered strong support for the monophyly of Archiheterodonta, within which Astartidae is the sister group of Crassatellidae, and these two constitute the sister clade of Carditidae, which is paraphyletic with respect to Condylocardiidae. The relationships among the constituent species groups were evaluated in the context of the archiheterodont fossil record through the estimation of divergence times. Diversification times of archiheterodont families were congruent with bounded estimates of origins based on palaeontological data: Archiheterodonta diversified during the Devonian, 373.1 Ma (95% highest posterior density interval [HPD] 325.8–428.2); Crassatelloidea around the Carboniferous, 330.1 Ma (95% HPD 291.0–372.7); Crassatellidae around the Triassic, 224.0 (95% HPD 140.6–320.2); Astartidae around the Permian, 288.2 Ma (95% HPD 269.2–307.3); and Carditoidea around the Jurassic, 178.8 Ma (95% HPD 120.9–228.3).