Phylogeny of the arachnid order Opiliones (Arthropoda) inferred from a combined approach of complete 18S and partial 28S ribosomal DNA sequences and morphology

The phylogenetic relationships among the main evolutionary lines of the ara chnid order Opiliones were investigated by means of molecular (complete 18S rDNA and the D3 region of the 28S rDNA genes) and morphological data sets. Equally and differentially weighted parsimony analyses of independent and combined data sets provide evidence for the monophyly of the Opiliones. In all the analyses, the internal relationships of the group coincide in the m onophyly of the following main groups: Cyphophthalmi, Eupnoi Palpatores, Dy spnoi Palpatores, and Laniatores. The Cyphophthalmi are monophyletic and si ster to a clade that includes all the remaining opilionid taxa (=Phalangida ). Within the Phalangida the most supported hypothesis suggests that Palpat ores are paraphyletic, as follows: (Eupnoi (Dyspnoi + Laniatores)), but the alternative hypothesis (Laniatores (Eupnoi + Dyspnoi)) is more parsimoniou s in some molecular data analyses. Relationships within the four main clade s are also addressed. Evolution of some morphological characters is discuss ed, and plesiomorphic states of these characters are evaluated using molecu lar data outgroup polarization. Finally, Martens' hypothesis of opilionid e volution is assessed in relation to our results. (C) 1999 Academic Press.