Evolution of the vertebrate jaw from developmental perspectives.

Attainment of the biting jaw is regarded as one of the major novelties in the early history of vertebrates. Based on a comparison between lamprey and gnathostome embryos, evolutionary developmental studies have tried to explain this novelty as changes in the developmental patterning of the mandibular arch, the rostralmost pharyngeal arch, at the molecular and cellular levels. On the other hand, classical theories in the field of comparative morphology assumed the involvement of hypothetical premandibular arch(es) that ancestral animals would have possessed rostral to the mandibular arch, in the transition from agnathan to gnathostome states. These theories are highly biased toward the segmental scheme of the vertebrate head, and the concept of premandibular "arches" is no longer accepted by the current understanding. Instead, the premandibular domain has now become of interest in the understanding of cranial development, especially in its rostral part. As newer theories that consider involvement of the premandibular domain, the neoclassical and heterotopy theories are here compared from evolutionary developmental perspectives, in conjunction with the development of nasal and hypophyseal placodes, in the context of the evolutionary acquisition of the jaw. Given recent advances in understanding of the lamprey development, evolution of the Dlx code is also discussed together with the evolutionary scenario of jaw acquisition.