Molecular phylogeny of the Bivalvia inferred from 18S rDNA sequences with particular reference to the Pteriomorphia

The classification of the Pteriomorphia. a major division of the Bivalvia with Ordovician origins, is controversial both among palaeontologists and neontologists. To elucidate phylogenetic relationships new near-complete 18S rDNA sequences of 26 Pteriomorphia. three Protobranchia, three Heterodonta, one Anomalodesmata and three Scaphopoda were obtained. aligned with 71 other published molluscan sequences, and analysed with parsimony, maximum likelihood and spectral analysis. Although Bivalvia appear diphyletic due to heterogeneity of substitution rates among lineages, monophyly of Protobranchia. Heteroconchia and Pteriomorphia is supported. The heteroconch Lucinida, Myoida and Venerida are not monophyletic, and Anomalodesmata arise from within Heteroconchia. The basal nodes of Pteriomorphia hate little support but two major clades. [Pinnoidea (Ostreoidea + Pterioidea)] and [(Anomioidea + Plicatuloidea) + (Limoidea + Pectinoidea)], are resolved with more confidence. The strongly supported clade of Anomioidea + Plicatuloidea, the separation of Pinnoidea from Pterioidea and most of the intrafamiliar relationships are not in accordance with morphological classifications. Combining these results with selected morphological characters a phylogenetic hypothesis is proposed showing Mytiloidea and Arcoidea as the basal pteriomorph groups. the latter giving rise to the clade uniting the pinnoid-ostreoid-pterioid and the anomioid-limoid-pectinoid lines.