Gadow's Arcualia and the Development of Tetrapod Vertebrae

Abstract

Critical reexamination of Gadow''s theory of the composition of vertebrae from four pairs of arcualia is inapplicable to tetrapods and so remote from the facts that his terminology even in modified form should not be employed in tetrapods. Division of the sclerotome into anterior and posterior parts by a sclerocoel is probably primitive in tetrapods. The origin of adult vertebral structures from these sclerotome parts is always complex. Centra in tetrapods are not arch centra, but autonomous structures, primitively two per neural arch, a hypocentrum formed perichordal tissue belonging to the cranial and caudal halves of a single sclerotome, and a pleurocentrum formed from perichordal tissue from the caudal half of one sclerotome united with perichordal tissue from the cranial half of the next succeeding sclerotome. The main body of the neural arch of tetrapods is either wholly derived from the paraneural tissue of the caudal half of a single sclerotome or from this plus paraneural tissue from the cranial half of the next succeeding sclerotome. The pre- and post-zygapophysial articulations form within the cranial halves of sclerotomes. Ventral arches, when present, are formed either from tissue from both halves of a single sclerotome or from one or the other half. The neural arch is homologous throughout the tetrapoda, living and fossil. The functional centrum, if there is a departure from the primitive condition with two functional centra, may be either the hypocentrum or the pleurocentrum. The evidence suggests that the hypocentrum became the functional centrum only in one branch of the tetrapoda which died out at the end of the Triassic and that all living tetrapods[long dash]the three orders of Amphibis and the Amniota[long dash]retain the pleurocentrum as the functional element. It is therefore suggested that the living Amphibia, on the basis of vertebral structure, are more closely akin to the Amniota than to the Paleozoic Amphibia termed Rhachitomi and Stereospondyli.