Gnathostome vertebrae and the classification of the Amphibia

Four pairs of arcualia were primitively present in each segment of gnathostomes. The individual vertebral ossifications of early temnospondyls are most economically interpreted as the endochondral ossifications of these cartilaginous arcualia. Centra have formed independently on at least two occasions within the tetrapods and arcualia play little or no part in the formation of true centra in any living form. The so called pleuro- and intercentra of the temnospondyls can in no way be homologized with the centra of either lissamphibians or amniotes. The Nectridea are considered to be the sister group of the Lissamphibia and the Aistopoda the sister group of these two. The anthracosaurs, seymourians and microsaurs are regarded as amniotes. There is no evidence for resegmentation in the vertebral column.