Chapter 7: Tooth vs. Beak: The evolutionary developmental control of the avian feeding apparatus

Recent discoveries of exquisitely preserved nonavialan and avialan theropod dinosaurs have not only prompted studies of theropod tooth morphologies, but have also provided information about the origin and early evolution of avian beaks. Recent studies on beak morphologies and morphogenesis in Darwin's finches have greatly improved our understanding of how avian beaks adapt to various ecological niches, but the question of how birds lost their teeth during the course of evolution has long been debated. Evolutionary developmental experiments performed on extant bird embryos bridge the gap between paleontological and neontological evidence, suggesting that the avian beak could have originated through heterochronic truncation of odontogenesis over evolutionary time. Here, we systematically review independently evolved regional and complete edentulism present in nonavialan and avialan theropod dinosaurs, and suggest that the tooth-reduction processes of different jaw bones are likely to be independently controlled. Through reviewing the recent advances of molecular regulations involved in tooth and avian beak morphogenic processes, we suggest that several molecules regulating the development of the avian beak also mediate the growth of keratinous rhamphothecae, and the divergence of odontogenic signalling pathways are likely to have accounted for both of these processes.