Developmental aspects of lepospondyl vertebrae in Paleozoic tetrapods

Abstract

The distinction between the two major patterns of vertebral structure among Paleozoic tetrapods may be the result of the emphasis on different developmental tissues. The spoolshaped centra of lepospondyls and primitive amniotes may have developed initially from the perichordal tube, external to the notochord, as is the case for modern tetrapods and teleosts. The multipartite centra of labyrinthodonts may have formed from the more medially located fibrous sheath of the notochord, by analogy with the chordacentra which are important elements in the developing centra of Amia and Lepisosteus. The transition between the two patterns may be attributed to selection for more rapid ossification and retention of a minimally restricted notochord during development, as well as the advantages of a solid, spoolshaped structure in the adult. The transition may have occurred through elaboration of spoolshaped chordacentra which were subsequently supplanted by the earlier initiation of perichordal ossification. Holospondylous centra probably evolved through the same processes in lepospondys land amniotes, but the distinct structure of the anterior cervical vertebrae indicates that it occurred separately in these two groups, as it did in teleosts.