Constraints on primate craniofacial growth and architecture

This study used 3-D morphological data, from both CT scans and radiographs, to test for the existence of constraints acting on the face, anterior cranial base and brain in primates. These data show that primate skulls, while composed of numerous separate units related to specific organs or spaces, are constrained by the need to integrate these units as functional systems during development and evolution.; Bivariate and multivariate scaling analyses were used to test several sets of hypotheses about constraint on skull form. First, a series of hypotheses about capsular matrices were tested, including whether soft tissues and spaces adjacent to the cranial base (the brain, its components, and the eyeball) constrain its form; whether the eyeball restricts size and shape of the nasomaxillary complex; and whether capsular matrices associated with the cranial base constrain size and shape of the nasomaxillary complex through integration among the brain, cranial base and face. Second, thirteen linear and three angular hypotheses about spatial relationships among the skeletal units themselves, irrespective of capsular matrices, were tested. Third, the influence of brain and basicranial growth on basicranial flexion was tested for ontogenetic samples of six primate species.; The above hypotheses were tested using (1) an adult interspecific CT sample of 45 extant primate species, including 15 strepsirrhines and 30 haplorhines; (2) an ontogenetic, radiographic sample of 6 species; and (3) soft tissue data taken from the literature.; Results indicate that several capsular matrices, including the brainstem, olfactory bulb, and temporal and frontal lobes, constrain sizes and shapes of those parts of the cranial base and nasomaxillary complex with which they are in direct contact, but have minimal interactions with more distantly-located parts of the skull, including the mandible. The eyeballs, on the other hand, do not constrain sizes or shapes of the cranial base or nasomaxillary complex. Interactions between skeletal tissues in the basicranium, nasomaxillary complex and mandible are common, but not as pervasive as modeled in some hypotheses; in fact, two of the angular constraints proposed to be invariant across mammals are not maintained among primates. The orientation of the anterior nasomaxillary complex, however, is essentially perpendicular to the orientation of the cribriform plate across primate species. Finally, humans scale differently from other primates for the relationship between relative brain size and cranial base flexion during growth, suggesting that spatial packing constraints are not as serious as previously conjectured for Homo sapiens.; In conclusion, although constraints may not be as pervasive as previously conjectured, a few key constraints may be extremely important in the evolution of primate and human skull form.