Phylogeny of the Rhabdocoela (Platyhelminthes): a working hypothesis

Abstract

An attempt is made to resolve contradictions in recent studies of the phylogeny of the Rhabdocoela (including the major parasitic flatworm groups), with a hypothesis based on parsimony analysis of characters determined at both the light microscopic and the ultrastructural level. The rhabdocoel subtaxa "Dalyellioida" and "Typhloplanoida" are shown to be non-monophyletic. The taxon Endoaxonemata, comprising the last common ancestor of the Neodermata, Pterastericolidae, Fecampiidae, and Acholadidae and all its descendants is named as a result of the study. Crustaceans are proposed as the primitive hosts of this group. Separate islands of equally parsimonious trees were encountered in some of the parsimony analyses performed. Failure to find all such islands may cause misinterpretation of the results. The impact on the results of different methods of rooting cladograms and different heuristic search strategies implemented in the PAUP software are discussed.