Three-dimensional kinematics of skeletal elements in avian prokinetic and rhynchokinetic skulls determined by Roentgen stereophotogrammetry

Several different types of cranial kinesis are present within modern birds, enabling them to move (part of) the upper bill relative to the braincase, This movement of the upper bill results from movement of the quadrate and t he pterygoid-palatine complex (PPC). The taxon Palaeognathae is characteris ed by a very distinct PPC and a special type of cranial kinesis (central ki nesis) that is very different from that found in the Neognathae. This has l ed some authors to hypothesise that there is a functional relationship betw een the morphology of the PPC and the type of cranial kinesis. This hypothesis is tested here by analysing the movement pattern of both th e upper bill and the PPC in birds with three different types of cranial kin esis: prokinesis, distal rhynchokinesis and central rhynchokinesis. Movemen t patterns were determined using a Roentgen stereophotogrammetry method, wh ich made it possible to detect very small displacements (0.5 mm) of bony el ements in three dimensions, while the jaw muscles and ligaments remained in tact, We found that in all types of kinesis investigated the movements of the qua drate, jugal bars and PPC are similar. Movement of the quadrate is transfer red to the upper beak by the jugal bar and the PPC, which moves almost excl usively forwards and backwards, thereby elevating or depressing the upper b ill, The differences between the types of kinesis lie only in the position of the point of rotation. These findings indicate that there is no correlation between the specific m orphology of the PPC and the type of cranial kinesis, Several other factors , including the external forces applied during food acquisition, may influe nce the morphology of the PPC, Differences in PPC morphology therefore appe ar to be the result of different functional demands acting on the system si multaneously but with different strengths, depending on the species.