On aspects of skull form in African apes and orangutans, with implications for hominoid evolution.

The study of hominoid phylogeny is currently in a state of controversy and debate due to the discovery of new fossil material and reanalysis of the morphology of extant apes. An important key to the resolution of these debates lies in attaining a fuller understanding of the morphological differences in skull form between the African and Asian great apes. In this paper I have analyzed aspects of facial morphology and internal cranial anatomy in the great apes. Results from this study and previous ones suggest that Pongo is characterized by a marked dorsal deflection of the face relative to the basicranium. Many aspects of circumorbital, midfacial, palatal, and mandibular morphology in Pongo may be related to this airorynchous condition. This hypothesis is supported by Enlow's work on form and pattern in the primate and mammalian skull. The position of the face in known Sivapithecus appears to be similar to that seen in Pongo. Although Pongo may be specialized in its marked degree of airorynchy, it seems likely that an important derived feature linking African apes and hominids is a ventral rotation of the splanchnocranium on the neurocranium. The appearance of marked supraorbital tori and ethmofrontal sinuses are probably correlated developments. Additional implications of this work for debates about hominoid phylogeny are discussed.