A functional analysis of the primate masticatory system and the origin of the anthropoid post-orbital septum.

Study of the dry weights of primate and non-promate masticatory musculature reveals a significant relationship between the Anterior Temporalis/Masseter ratio and the relative development of the anterior dentition. Available dietary information demonstrates that species emphasizing incisal preparation of food have a high AT/M index; species emphasizing molar occlusion have a low AT/M index. Utilizing this information, a model is presented of the origin of the anthropoid post-orbital septum. Frugivory or extensive incisal preparation of food is causally related to the development of the post-orbital septum, because diet can then create selection pressures for an increasingly tendinous and enlarging anterior segment of the temporalis muscle which requires additional bony areas of orgin in the anterior temporal fossa. Cenozoic climatic oscillations leading to increasing seasonality may have been the triggering element in this model, because seasonality creates periods in which the availability of fruit is relatively predictable.