Spatial packing, cranial base angulation, and craniofacial shape variation in the mammalian skull: testing a new model using mice.

The hypothesis that variation in craniofacial shape within and among species is influenced by spatial packing has a long history in comparative anatomy, particularly in terms of primates. This study develops and tests three alternative models of spatial packing to address how and to what extent the cranial base angle is influenced by variation in brain and facial size. The models are tested using mouse strains with different mutations affecting craniofacial growth. Although mice have distinctive crania with small brains, long faces, and retroflexed cranial bases, the results of the study indicate that the mouse cranial base flexes to accommodate larger brain size relative to cranial base length. In addition, the mouse cranial base also extends, but to a lesser degree, to accommodate larger face size relative to cranial base length. In addition, interactions between brain size, face size, and the widths and lengths of the components of the cranial base account for a large percentage of variation in cranial base angle. The results illustrate the degree to which the cranial base is centrally embedded within the covariation structure of the craniofacial complex as a whole.