Article: New evidence on the nature of the jaw suspension in Palaeozoic anacanthous sharks

Gegenbaur's classic and almost universally accepted view of the primitive visceral skeleton of vertebrates, envisioning gill, hyoid, and mandibular arches as uniform, serially homologous (homonomous) structures separated by gill clefts, has not been actually demonstrated among either recent or fossil forms. In all cases the mandibular and hyoid arches are specialized in various ways to meet the functional requirements of the mandibular arch that frames the mouth opening. Apparently correlated with these modifications of the first two arches is the loss of a fully developed gill slit between them. Search for a visceral skeleton in which a prehyoidean gill cleft is present (aphetohyoidean condition) has hitherto been conducted unsuccessfully among the acanthodians and placoderms. Such a structure is here reported to be present in certain sharks of Upper Carboniferous age, in which the visceral skeleton is intermediate between Gegenbaur's theoretical scheme and the most generalized condition hitherto recorded among fossil and modern vertebrates. These Palaeozoic elasmobranchs are hence the most primitive gnathostomes presently known.