Preliminary cladistic analysis corroborates the hypothesis that the Elasmobranchii and the Holocephali are sister groups, and that the Chondrichthyes are more closely related to the Teleostomi (Acanthodi plus Osteichthyes) than either is to the Placodermi. In regard to the Paleozoic and Mesozoic elasmobranchs, a theory of relationships has been proposed for six better known genera (Xenacanthus, Denaea, Cladoselache, Hybodus, Ctenacanthus and Paleospinax) by evaluation of certain characters in the skull, postcranial axial skeleton, fin supports, and fins.
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Schaeffer et al. (1977) studied this question.
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