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

148 YEARBOOK OF PHYSICAL ANTHROPOLOGY [Vol. 43, 2000
tion, and also to study potential interactions
between cranial base shape and facial shape
in primates and other mammals.
THE CRANIAL BASE AND POSTURE
Apart from a few recent studies reviewed
here, little is known about the relationship
between basicranial morphology, head and
neck posture, and other aspects of head
morphology related to locomotion. One persistent
hypothesis that is especially relevant
to hominin evolution is that flexion of
the cranial base is an adaptation for orthograde
posture in hominins because it causes
the foramen magnum to have a relatively
more anterior position and ventral orientation
(Bolk, 1909, 1910; Duckworth, 1915;
Weidenreich, 1941; Schultz, 1942, 1955;
Ashton and Zuckerman, 1952, 1956; Ashton,
1952; Moore et al., 1973; Adams and
Moore, 1975; DuBrul, 1977, 1979; Dean and
Wood, 1981, 1982). Indeed, these features
are often invoked in attempts to reconstruct
head posture in fossil hominins (e.g., White
et al., 1994). All primates have the center of
mass of the head located anterior to the
occipital condyles, such that more anteriorly
positioned occipital condyles relative to
head length reduce the lever arm between
the center of mass and the atlanto-occipital
joint. This balancing, in turn, reduces torque
about this joint, thereby reducing the magnitude
of the force required from the nuchal
muscles to hold up the head (Schultz, 1942).
The occipital condyles can be moved rostrally
relative to overall head length by flexing
the basicranium and/or shortening the
posterior cranial base. In vitro experiments
by Demes (1985) demonstrated that the
more ventral orientation of rostrally placed
occipital condyles orients the articular surfaces
closer to perpendicular to the compressive
force acting through the center of mass
of the head, potentially reducing shearing
forces acting across the joint that need to be
resisted by muscles or ligaments.
There are some experimental data that
rodents forced to walk bipedally (Moss,
1961; Fenart, 1966; Riesenfeld, 1966) develop
more flexed cranial bases. However,
the hypothesis that variations in cranial
base angle are adaptations for head posture,
however, is not well supported by comparative
data. Among primates, basicranial flexion
has been shown to be uncorrelated with
either qualitative estimates of body posture
(Ross and Ravosa, 1993) or quantitative
measures of head and neck orientation
(Strait and Ross, 1999). The partial correlation
analysis of Strait and Ross (1999) confirmed
relative brain size as a more important
determinant of variation in basicranial
angle, even when facial orientation and
head and neck posture were taken into account.
Although foramen magnum orientation
relative to anterior cranial base orientation
(S-FC) has been shown to be related
to relative brain size (Biegert, 1963; Spoor,
1997), the relationship between head and
neck posture and foramen magnum orientation
has not yet been evaluated.
We gathered data on foramen magnum
orientation (FM) from the same radiographs
used by Ross and Ravosa (1993) and Ravosa
(1991b), and combined these data with measures
of hominids reported by Spoor (1993)
and with measures of head and neck orientation
reported by Strait and Ross (1999)
(Fig. 13). The head-neck angle is the angle
between neck inclination and orbit inclination,
both relative to the substrate (Strait
and Ross, 1999). The values for FM orientation
relative to the clivus (FM CO) are in
Table 7 and are summarized in Figure 14.
The values for FM orientation relative to
the orbital axis were calculated from measures
of the orientation of both these planes
relative to the clivus ossis occipitalis (CO) as
180°-AOA-FM CO. These data show that
FM CO is not significantly correlated
with basicranial flexion, orbital axis orientation,
the orientation of the head relative
to the neck, or the size of the cerebellum
relative to the posterior basicranium (Table
8). Nor is foramen magnum orientation relative
to the orbital axis (FM AOA) correlated
with any of these variables, except
AOA (Table 8). Of particular interest is the
lack of correlation between the head-neck
angle of Strait and Ross (1999) (Fig. 14),
suggesting that foramen magnum orientation
is not a good indicator of the orientation
of the neck during habitual locomotion. This
calls into question attempts to estimate
head and neck posture from data on foramen
magnum orientation in fossils.