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The phylogeny of the extant chelicerate orders is examined in the light of morphological and molecular evidence. Representatives from each of the chelicerate 'orders' and mandibulate and onychophoran outgroups are examined. Molecular (small and large ribosomal subunit DNA) and morphological information is combined in a total evidence regime to determine the most consistent picture of extant chelicerate relationships for these data. Multiple phylogenetic analyses are performed with variable analysis parameters yielding largely consistent results. A normalized incongruence length metric is used to assay the relative merit of the multiple analyses. The combined analysis with lowest character incongruence yields the scheme of relationships (Pycnogonida+ (Xiphosura+((Opiliones+((Solifugae+Pseudoscorpiones)+Scorpiones))+((Ricinulei+Acari)+(Palpigradi+ ((Thelyphonida+Schizomida=Uropygi)+(Amblypygi+ Araneae))))))). This result is fairly robust to variation in analysis parameters, with the placement of solifugids and the status of the pedipalps responsible for most disagreement.
Wheeler et al. (Mon,) studied this question.