Form, function, development and evolution of intraoral food processing in salamanders

In order to survive, animals have to eat - at least during certain phases of life, since the energy required for almost all life processes comes from the assimilation of existing organic compounds. Intraoral food processing as one of the four stages of feeding refers to any mechanical reduction or food preparation in the oral cavity before swallowing. Intraoral food processing involves rhythmic, cyclical, and usually coordinated movements of the skull, mandible, and tongue (i.e., the feeding apparatus). Jawed fishes (chondrichthyans, actinopterygians, and dipnoi) mostly use their mandible to process food, and hence this mechanism qualifies as chewing. The general pattern of these movements is mainly conserved across fishes. However, along with the transition from water to land during early tetrapod evolution, aquatic fish-like food processing (i.e., chewing and hydrodynamic food transport) has been replaced by terrestrial amniote-like food processing (i.e., chewing and tongue-based food transport). Although intraoral food processing occurs amongst almost all major groups of jawed vertebrates, it had been argued that recent amphibians mostly lack intraoral food processing mechanisms. The thesis at hand aims to expand our knowledge of the ontogeny and evolution of intraoral food processing in salamanders. I demonstrate that salamanders commonly use intraoral food processing and that the mechanism of food processing seems to switch from mandible-based chewing in larval morphotypes to a tongue-based palate rasping in metamorphic morphotypes. The ontogenetic switch in intraoral food processing that occurs in metamorphosing salamanders might be argued to resemble an analogue to the phylogenetic shift that happened during the evolution of early tetrapods. This analogue might suggest that direct tongue-based interactions with food have evolved under aquatic conditions hence, suggesting that terrestrial style feeding might have preceded the water-land transition.