Cf. Ross et Mj. Ravosa, BASICRANIAL FLEXION - RELATIVE BRAIN SIZE - AND FACIAL KYPHOSIS IN NONHUMAN-PRIMATES, American journal of physical anthropology, 91(3), 1993, pp. 305-324
Numerous hypotheses explaining interspecific differences in the degree
of basicranial flexion have been presented. Several authors have argu
ed that an increase in relative brain size results in a spatial packin
g problem that is resolved by flexing the basicranium. Others attribut
e differences in the degree of basicranial flexion to different postur
al behaviors, suggesting that more orthograde animals require a ventra
lly flexed pre-sella basicranium in order to maintain the eyes in a co
rrect forward-facing orientation. Less specific claims are made for a
relationship between the degree of basicranial flexion and facial orie
ntation. In order to evaluate these hypotheses, the degree of basicran
ial flexion (cranial base angle), palate orientation, and orbital axis
orientation were measured from lateral radiographs of 68 primate spec
ies and combined with linear and volumetric measures as well as data o
n the size of the neocortex and telencephalon. Bivariate correlation a
nd partial correlation analyses at several taxonomic levels revealed t
hat, within haplorhines, the cranial base angle decreases with increas
ing neurocranial volume relative to basicranial length and is positive
ly correlated with angles of facial kyphosis and orbital axis orientat
ion. Strepsirhines show no significant correlations between the crania
l base angle and any of the variables examined. It is argued that prio
r orbital approximation in the ancestral haplorhine integrated the med
ial orbital walls and pre-sella basicranium into a single structural n
etwork such that changes in the orientation of one necessarily affect
the other. Gould's (''Ontogeny and Phylogeny.'' Cambridge: Belknap Pre
ss, 1977) hypothesis, that the highly flexed basicranium of Homo may b
e due to a combination of a large brain and a relatively short basicra
nium, is corroborated.