NORENBURG, J.L.; THOLLESSON, M.; Natl. Mus. Nat. Hist., Washington DC; Uppsala Univ., Sweden: Diversification in the phylum Nemertea in light of a molecular phylogeny.

A recent molecular phylogeny of Nemertea, based on Bayesian analysis of partial sequence data from four genes for 55 species-level taxa, proposed three new higher-level clades and resolution for several contested clades (Thollesson & Norenburg 2003, Proc. R. Soc. Lond. 270:407-415). The largest new clade is Neonemertes, combining for the first time Hetero- and Hoplonemertea as sister taxa. Previous suggestions are upheld that Hubrechtella is a sister-taxon to the Heteronemertea s.l., their synapomorphy recognized in the new clade name Pilidiophora. Polystilifera is monophyletic, and Pelagica s.s. is monophyletic within a paraphyletic Reptantia. Thus, the pelagic habit is derived for Pelagica. Cratenemertidae comprises a monophyletic sister taxon to the remaining Monostilifera. The latter group now named Distromatonemertea in reference to its synapomorphy, a bilayered rhynchocoel wall. Interwoven rhynchocoel musculature of Polystilifera and Cratenemertidae may be plesiomorph for Hoplonemertea. Stiliferous proboscis armature is inferred to be monophyletic but ontogeny of poly- and monostilifery remains unresolved. Malacobdella, a much debated group of mollusk commensals, is solidly within the Monostilifera. The Palaeonemertea are basal, as implied by the name, but they are paraphyletic. Cephalothrix, Tubulanus and Poseidonemertes are the only species-rich genera in this analysis to remain monophyletic. Most traditional genera and families appear to be without merit, as has been argued in several recent morphological studies. Molecular results clash especially with morphological analyses for morphology-poor groups, such as commensal and pelagic nemerteans. Life-history and morphological characters are mapped and reveal a compelling hypothesis of the phylum�s diversification.