GNATHOSTOMULIDA - AN ENIGMATIC METAZOAN PHYLUM FROM BOTH MORPHOLOGICAL AND MOLECULAR PERSPECTIVES

On the basis of few and contentious morphological characters Gnathosto mulids have been thought to be the sister-group of either the Platyhel minthes or the Syndermata (Rotifera + Acanthocephala). We provide a fu ll 18S rDNA sequence for a species of Gnathostomula and attempt to res olve its position among the Metazoa, on the basis of molecular evidenc e. Sixty sequences, representing 30 nominal phyla and including new en toproct and gastrotrich sequences, were used to reconstruct phylogenie s using maximum-parsimony, neighbor-joining, and minimum evolution mod els. We were unable to support either of the morphological hypotheses outright and, moreover, our data supported more strongly a third possi ble relationship with the gnathostomulids as a member of the Nematoda + Chaetognatha clade. Superficially, as active benthic, vermiform crea tures with sclerotized cuticular jaws, they fit a predicted ancestral form of the Nematoda + Chaetognatha clade and, as such, would arguably be members of the Ecdysozoa. The molecular data at least call for a r eevaluation of the morphological data and a denser sampling of the les ser phyla. Data from morphology and molecules act synergistically in e stimating phylogeny; morphology alone provided limited phylogenetic si gnal and alternative phylogenetic hypotheses, whereas the molecular so lution suggested an alternative topology which, when interpreted in th e light of comparative anatomy, may suggest previously unconsidered po ssibilities. (C) 1998 Academic Press.