THE POSITION OF ARTHROPODS IN THE ANIMAL KINGDOM - A SEARCH FOR A RELIABLE OUTGROUP FOR INTERNAL ARTHROPOD PHYLOGENY

Morphological evidence for the phylogeny of the animal kingdom has bee n discussed by numerous authors. DNA sequencing and phylogenetic metho ds for analyzing these data are alternative approaches to animal phylo geny, but the phenomenon of long branch attraction and poor taxonomic sampling have caused misinterpretations of metazoan relationships. Her e we report a cladistic approach to metazoan evolution including 133 1 8S rDNA sequences of 31 animal phyla. Despite the difficulties associa ted with analyzing large data sets, our data suggest that the Bilateri a and Protostomia are monophyletic. The internal phylogeny of the prot ostomes is divided into two main clades. One clade includes the classi cal protostome worms (annelids, sipunculans, echiurans, pogonophorans, and vestimentiferans), mollusks, nemerteans, ''Iophophorates,'' platy helminths, rotiferans, and acanthocephalans, although the internal res olution of the clade is very low. The second clade includes arthropods and other molting animals: tardigrades, onychophorans, nematodes, nem atomorphans, kinorhynchs, and priapulans. The arthropods and related p hyla lack a ciliated larvae, lack a multiciliate (locomotory) epitheli um, and share many features, notably, a reduced coelomic cavity and th e presence of a cuticle which molts. The use of these outgroups within the molting clade to root arthropod phylogenies is recommended instea d of using annelids or other spiralians. The data are quite conclusive in those phyla with a good taxonomic sampling (i.e., platyhelminths a nd arthropods). (C) 1998 Academic Press.