44.3 Thursday, Jan. 5 Beyond the Beak: Modeling avian cranial kinesis and the evolution of bird skull shapes OLSEN, A.M.*; WESTNEAT, M.W.; University of Chicago, IL; Field Museum of Natural History, Chicago, IL email@example.com
Bird beaks display remarkable morphological, functional and mechanical diversity. While there is considerable literature on the evolutionary morphology of bird beaks and the relationship between beak morphology and ecology, the beak itself (the upper and lower bill) constitutes only two of nine links in the avian jaw apparatus. Cranial kinesis, or movement of the upper bill, depends on kinetic bones behind the upper bill that form four- and five-bar linkage mechanisms, or closed loops of interjointed bones. The upper bill, jugal, quadrate and skull comprise the four-bar linkage while the five-bar linkage includes the upper bill, palatine, pterygoid, quadrate and skull with the palatine or a palatine-pterygoid complex moving as a sliding link. Since the position of one link in a linkage mechanism is dependent on the position of all the other links, biological linkage mechanisms are particularly informative for inferring the mechanical properties of musculoskeletal systems from morphology. We have developed a three-dimensional linkage model of avian cranial kinesis that predicts the kinematics of the beak and its associated linkage bones as well as the distribution of forces throughout the mechanism by static force analysis. We have found that morphological diversity of the avian jaw extends beyond the beak to include the linkage bones underlying the beak. Using our model, we predict that this geometric diversity of the entire cranial linkage mechanism results in mechanical diversity not apparent from considerations of beak morphology alone. Placing these results into a phylogenetic context will provide insights into the evolution of beak function and to what extent cranial bones other than those of the upper and lower bill have influenced beak evolution. Supported by NSF GRFP to AMO and NSF DEB-0844745 to MWW.