The skull development of parrots with special reference to the emergence of a morphologically unique cranio-facial hinge.

The order Psittaciformes (parrots) has unique morphological features in the head that are evolutionarily novel. To better understand the unique evolution of the head in parrots, the developmental pattern of the skull of the budgerigar (Melopsittacus undulatus) was initially described on the basis of transparent skeletal specimens. Although the fundamental pattern of the skull development of birds is conserved in parrots, some differences were observed between parrots and other groups of birds. In parrots, the vacuity in the interorbital septum did not emerge throughout ontogeny, in contrast to other lineages of birds, for example Galliformes and Coliiformes. This feature seems to be concerned with the attachment of the unique jaw muscle of parrots, M. ethmomandibularis, to the interorbital septum. In spite of a prokinetic skull, the cranio-facial hinge of parrots was brought about by secondary transformation of dermal bones unlike that of birds with a standard prokinetic skull (e.g. Corvus) in which the nasal-frontal suture directly becomes a hinge of bending. To further understand the evolution of "pseudoprokinesis" in parrots, the construction of a robust avian phylogeny is desired. The parrot-specific suborbital arch and cranio-facial hinge are not seen until birds leave the nest and can feed themselves. In conclusion, these structures are considered to be essential for eating hard and/or large meals.