The primate cranial base: ontogeny, function and - Harvard University

144 YEARBOOK OF PHYSICAL ANTHROPOLOGY [Vol. 43, 2000
Fig. 12. Proposed “facial block,” showing the effects of angular invariance between the back of the face
(summarized by the PM plane) and the top of the face, which is also the bottom of the anterior cranial
base (S-FC). Changes in cranial base angle cause the top and back of the face to rotate together around
an imaginary axis through the PM point. See text for details.
1985a, 1988; Ross and Henneberg, 1995).
This airorhynchy has yet to be explained
developmentally and functionally; however,
Ross and Henneberg (1995) suggested that
it evolved in hominoids in response to increased
flexion of the cranial base producing
posterior displacement of the palate.
More research is needed on the integration
of the midface and cranial base. In particular,
why is the PM plane oriented at 90°
relative to the NHA during postnatal ontogeny
and thus across taxa? Another question
of interest is, what aspects of cranial base
and facial shape are responsible for most of
the variation in PM plane position, and
hence facial orientation? This problem has
not been well studied, but the orientation of
the PM plane is probably most affected by
the size of the middle cranial fossa, especially
the length of the temporal lobes, by
flexion of the sphenoid, and by the length of
the anterior sphenoid in the midline cranial
base. Three-dimensional studies of the interface
between the PM plane and the cranial
base are needed to resolve these and
other questions about cranial base-face interrelations
and interactions.
Basicranial width and overall facial
shape in humans
Although it is clear that the cranial base
plays a major role in influencing facial orientation
relative to the neurocranium, there
is less information about the potential influence
of the cranial base on other aspects of
facial shape such as height, length, and
width. To what extent is overall facial shape
independent of the cranial base? It is commonly
assumed that the majority of facial
growth is independent of cranial base
growth, largely because much of the face
grows in a skeletal growth trajectory after
the end of the neural growth phase. In humans,
for example, the face attains 95%
adult size by 16–18 years, at least 10 years
after the cranial base reaches adult size
(Stamrud, 1959; Moore and Lavelle, 1974).
In addition, most facial and basicranial dimensions
appear to be genetically independent
in adults (Cheverud, 1996). However,
there is some evidence to suggest that
changes in the proportions of the cranial
base can influence facial shape. This interaction
is predicted to be especially important,
and perhaps exclusive to humans, in
which the upper face lies almost completely
underneath the anterior cranial fossa (Weidenreich,
1941; Howells, 1973; Enlow and
Bhatt, 1984; Enlow, 1990; Lieberman et al.,
2000).
The most explicit of these hypotheses is
that of Enlow (1990), who suggested that
humans with absolutely narrow cranial
bases (primarily dolichocephalics) tend to