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

162 YEARBOOK OF PHYSICAL ANTHROPOLOGY [Vol. 43, 2000
Constraint. Another expected outcome of
integration is constraint, which is defined
most generally as a restriction or limitation
on variation. Regardless of their causes,
which can be phylogenetic, functional, developmental,
or structural (Alberch, 1985;
Maynard Smith et al., 1985), constraints are
most basically evident in patterns of invariance.
There has not been much work on
phenotypic constraint in the cranial base,
but several examples suggest it deserves
further research. One source of evidence for
constraint is revealed by allometry, which
measures size-related conservation of shape
(keeping in mind that size-correlated patterns
due to epigenetic and perhaps genetic
factors differ from size-required patterns
that have a primarily functional basis). As
Table 4 (see also Strait, 1999; McCarthy,
2001) illustrates, there are numerous,
strongly correlated ontogenetic scaling relationships
across primates between various
components of the cranial base, the volume
of neural regions, and facial dimensions. Integration
between the brain and face via the
cranial base is implied by the fact that many
scaling relationships between contiguous
anatomical units (e.g., the noncortical brain
and the posterior cranial base) result in additional
strongly correlated scaling relationships
between noncontiguous anatomical
units, such as those between brain volume
and facial size. These scaling relationships
require more study.
Another potential source of evidence regarding
the presence of a constraint are patterns
of angular invariance. One important
example is the apparently invariant 90° angle
between the PM plane and the NHA,
and the limited variability that results from
this relationship on the angle between the
PM plane and both the planum sphenoideum
and the anterior cranial base (S-FC)
in anthropoids (McCarthy and Lieberman,
2001). Other less secure examples of invariance
may include the relationship between
cribriform plate orientation and facial orientation
(Ravosa and Shea, 1994), and the
near 45° angle between the external auditory
meatus, the maxillary tuberosities, and
the midpoint of the orbital aperture (Bromage,
1992). Further research, however, is
needed on the extent to which invariant angles
and spatial relationships occur in the
skull, and additional research is needed to
assess the developmental, structural, and
functional bases of these relationships, and
whether they reflect constraints that result
from integration.
Ontogenetic sequence. Finally, hypotheses
of phenotypic integration may sometimes
be inferred or tested by examining
ontogenetic sequences during normal
growth and in the context of controlled experiments.
Ontogenetic data allow one to
test hypotheses of integration by examining
the structural relationships between one
variable and another during growth (e.g.,
heterochrony, heterotopy), and to compare
the pattern and timing of developmental
events. So far, there have been few attempts
to examine hypotheses of integration in the
cranial base using ontogenetic data (especially
from embryonic stages), but a few examples
indicate that the sequence of interactions
between the cranial base, the face,
and the brain are complex and multiphasic,
with the cranial base mediating various interactions
between the face and the brain.
For example, the prenatal human cranial
base initially flexes, then remains stable,
and then extends, all during periods of rapid
neural growth (Jeffery, 1999). Postnatally,
the nonhuman primate cranial base (best
studied in Pan and Macaca) appears to extend
slightly during the period of neural
growth, and subsequently extends more
rapidly and for a long time as the face continues
to grow. In contrast, the human cranial
base flexes rapidly during the first few
years of brain growth, and subsequently remains
stable. These contrasting sequences
imply multiple interactions between the
brain and the face via the cranial base, but
have yet to be resolved in terms of the actual
processes that cause flexion and extension
at specific locations during different periods
of growth. One hypothesis, which remains
to be tested, is that the cranial base functions
to accommodate and perhaps coordinate
these different aspects of growth. Controlled
experimental studies, which can
potentially isolate local and regional effects
of specific growth stimuli on the cranial base
and the rest of the skull, are a promising