YOUNG, Nathan M*; WAT, Stephen; DIEWERT, Virginia; HALLGRIMSSON, Benedikt; University of Calgary; University of Calgary; University of British Columbia; University of Calgary: The Relationship of Embryonic Facial Size to Adult Facial Length

Facial length is an evolutionarily labile trait in mammals whose expression has numerous functional implications. There are many possible developmental mechanisms for variation in this trait, including responses to hormones or mechanical loading during ontogeny, patterns of postnatal growth, and initial size of the embryonic anlage. Here we test the hypothesis that embryonic facial size is correlated with adult facial length in strains of mice which are known to vary in this trait. The sample consists of facial landmarks in adults (90 days) and embryos (11 days postfertilization corrected for number of tail somites) of several strains of mice. Crania of adult mice were scanned using a �CT and then landmarked in three dimensions using ImageJ. Embryos were scanned either as above or by photographing in lateral and frontal view and then landmarking in two dimensions. Geometric morphometrics was used to quantify, compare, and visualize shape variation between strains. Sizes of the facial prominences and adult facial length were calculated from the landmarks. Support for this hypothesis indicates that processes affecting embryonic maxillary mesenchymal proliferation rate are potential targets for evolutionary transformations associated with increases or decreases in the length of the face. Results from this analysis have important implications for the evolution of both reductions and increases in facial length (e.g., in humans and papionin primates, respectively).