A molecular phylogeny of the bivalve mollusks.

A phylogenetic reconstruction based on 506 nucleotides near the 5' end of the 18S subunit of ribosomal DNA (rDNA) in 2 gastropod, 3 chiton and 28 bivalve mollusks supported the monophyly and sister group relationship of the subclasses Heterodonta and Palaeoheterodonta but could not confidently establish either the monophyly or the phylogenetic relationships of the morphologically well defined subclasses Pteriomorphia, Protobranchia, and Anomalodesmata. When both gastropods and chitons were included in the analysis, one or the other invariably emerged within Bivalvia. Some evidence indicates that this apparent polyphyly may be the consequence of unequal rates of evolution and of rapid changes in the protobranch and anomalodesmatan lineages. The taxa usually included in Pteriomorpha emerge as a grade rather than a clade, although in a sequence that differs from morphologically based phylogenies.