The ontogeny of Pan troglodytes craniofacial architectural relationships and implications for early hominids

Developmental constraints characterize functional growth boundaries of the mammalian face. Enlow and colleagues identified boundary conditions and planes of the midface in humans and other mammal species that are defined on the basis of important growth sites and the developmental disposition of neural and pharyngeal matrices. With few exceptions a conservative mammalian architecture is said to be achieved by the adult stage. Three aspects of this architecture are investigated here for a cross-sectional ontogenetic series of Pan troglodytes crania: (1) a line passing from the maxillary tuberosity through the junction of middle and anterior cranial fossae is perpendicular to the neutral horizontal axis of the orbit (PM-NHA angle), (2) an average 45° angle, whose origin is the external auditory meatus, separates the maxillary tuberosity from the midpoint of the orbital opening (meatus angle) and (3) the base of the brain, maxillary tuberosity and prosthion are on or close to the same plane (anterior maxillary hypoplasia). This investigation considers to what extent samples of chimpanzees and early hominids reflect Enlow's characterization of mammalian craniofacial architecture. Evidence derived from the chimpanzee sample bears out the perpendicularity of the PM-HNA angle and a near 45° meatus angle (though slightly higher overall), but does not conform to the absence of marked anterior maxillary hypoplasia seen in other mammals. Pan is instead characterized by considerable ontogenetic variation (noted by Enlow amongst anthropoids) influenced to some extent by sex. Preliminary early hominid data on the meatus angle indicates that Australopithecus specimens are similar to the widespread and probably primitive condition, while Paranthropus and early Homo specimens portray an uncharacteristic mammalian architectural relationship.