PALEONTOLOGICAL AND MOLECULAR EVIDENCE ON BIVALVE PHYLOGENY

Increasing availability of molecular data and new studies on fossil material are improving our understanding of bivalve phylogeny, but few studies integrate these data sets. The known Cambrian bivalves appear to represent the stem group, but many extant subclasses and orders appear by the lower Ordovician. Current molecular data do not provide good resolution of the relationships among these taxa, which may support rapid radiation. However, several key taxa, especially protobranchs, remain unstudied. Paleontological data support the derivation of solemyoids from ctenodontids. The Pteriomorphia are generally supported as monophyletic by molecular and morphological evidence. Cyrtodonts probably represent the stem group of the Pteriomorphia. The relationships between the pteriomorph orders are not all resolved. Close relationships between oysters and pearl oysters and between anomioideans and plicatulids are supported by both fossil and molecular evidence. The latter seem to belong, along with limids, in Pectinoida. The remaining autobranchs have moderate support as a monophyletic Heteroconchia from both paleontological and molecular data. The actinodonts are the stem group for Heteroconchia. Four extant clades can be traced back to the middle or lower Paleozoic. Trigonioids and unionoids derive from lyrodesmatids. Crassatelloideans and carditoideans are excluded from Veneroida by molecular data. They probably derive from Eodon and may be related to the modiomorphids. Modern anomalodesmatans are the sister taxon of Veneroida. Several Paleozoic taxa have been assigned to Anomalodesmata, but their status as true anomalodesmatans versus stem heteroconchs or extinct clades remains unclear. Molecular analyses place Lucinoidea (excluding Ungulinidae) as the basal stock of Veneroida, in agreement with their Silurian fossil record. Myoids are polyphyletic, derived at least three times from Veneroida.