The position of arthropods in the animal kingdom: Ecdysozoa, islands, trees, and the "Parsimony ratchet".

In a recent paper published in Molecular Phylogenetcs and Evolution, Giribet and Ribera (1998) placed rthropods within the Animal kingdom by analyzing he 18S rDNA locus of 133 metazoan taxa. The goal of hat paper was twofold: (1) to investigate the position of he Arthropoda within the protostome animals and (2) o determine arthropod sister-group relationships. Seuence data were analyzed by means of unweighted arsimony (gaps coded as a character state) in NONA .5.1 (Goloboff, 1993) using different tree-searching trategies, with the combination of different swappers. iribet and Ribera found 756 trees of 7032 steps. A trict consensus of these 756 trees is shown in Fig. 1A. he phylogenetic conclusions were summarized as folows: (1) The Bilateria are supported as monophyletic. (2) The Mesozoa appear as a monophyletic group and re the sister group to the remaining Bilateria. Other onophyletic groups are Ctenophora, Placozoa, Cniaria, Urochordata, Craniata, Echinodermata, Enteroneusta, Priapula, Tardigrada, Nematoda, Nematomorha, Onychophora, Arthropoda, Platyhelminthes, litellata, Sipuncula, Ectoprocta, Pogonophora (includng Vestimentifera), and Acanthocephala. Nevertheess, Mollusca,Annelida, Phoronida, Brachiopoda, Bryooa, Nemertea, and Rotifera appear as nononophyletic groups. (3) The Deuterostomia appear as a paraphyletic roup. (4) The Protostomia appear monophyletic and split nto two main lines: (1) a clade constituted by animals ith typical spiralian development plus some ‘‘aschel-