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

D.E. Lieberman et al.]
PRIMATE CRANIAL BASE 151
below the orbital axis line). These data
therefore lend support to the hypothesis
that animals orient their heads at rest (and
during locomotion) primarily to orient their
eyes towards the horizon rather than to orient
their semicircular canals in any particular
way. In sum, orbital axis orientation
relative to the clivus and foramen magnum
orientation relative to the orbital axis are
hypothesized to be evolutionary adaptations
to head posture during habitual locomotion,
and appear necessary because of the limited
range of motion at the atlanto-occipital joint
(Graf et al., 1995).
MAJOR UNRESOLVED ISSUES OF
CRANIAL BASE VARIATION IN PRIMATE
EVOLUTION
Studies of cranial base variation in fossil
primates and hominins have been rare because
this region of the skull is usually
poorly preserved or destroyed in most fossils,
and because it is hard to visualize or
measure without radiographs or computed
tomography (CT) scans. The coming years,
however, are likely to see a renaissance of
research on the role of the cranial base in
primate cranial evolution as CT scans of
fossils become more readily available. Here
we review six major topics where future research
on the cranial base in both fossil and
extant primates promises to provide important
insights: 1) the relationship between
encephalization, circumorbital form, and
the origin of primates; 2) the evolution of
the integrated “facial block” in haplorhines;
3) the determinants of basicranial flexion in
hominins; 4) the relationship between basicranial
flexion and the shape of the vocal
tract; 5) the role of the cranial base in facial
retraction in Homo sapiens; and 6) the reliability
of basicranial characters as indicators
of primate phylogeny.
Primate origins, encephalization, and
circumorbital form
The basicranium likely played a key role
in the evolution of the unique configuration
of the primate skull. Over the past three
decades, the visual predation hypothesis
(VPH) has become a well-accepted model of
primate origins (Martin, 1990; Fleagle,
1999). The VPH argues that the first primates
were nocturnal visual predators of
small invertebrates and vertebrates, and
this required more anteriorly facing and
closely approximated orbital apertures
(Cartmill, 1970, 1972, 1974, 1992). Increased
orbital convergence enlarges the
binocular field for greater stereoscopic vision
and a clear retinal image for depth
perception and prey location (Allman, 1977,
1982). The VPH further posits that relatively
larger orbits and grasping appendages
with nails are adaptations to being
nocturnal in an arboreal, terminal-branch
setting (Cartmill, 1970, 1972, 1974, 1992;
Kay and Cartmill, 1974, 1977; Dagosto,
1988; Covert and Hamrick, 1993; Hamrick,
1998, 1999; Lemelin, 1999). These adaptations
differ significantly from the cranial
and locomotor specializations of putative
sister taxa such as plesiadapiforms and dermopterans
(Cartmill, 1972, 1974, 1992; Kay
and Cartmill, 1974, 1977; Beard, 1993; Ravosa
et al., 2000a).
According to the VPH, increased orbital
convergence moves the orbital apertures out
of the plane of the temporal fossa, a condition
entailing greater ocular disruption during
mastication (Cartmill, 1970, 1972, 1974,
1992). A rigid postorbital bar may function
to stiffen the lateral orbital margins and
thus counter ocular deformation during biting
and chewing to ensure a high level of
stereoscopic acuity in an organism that processes
food while hunting and foraging
(Cartmill, 1970, 1972). This appears to be
particularly important, given that strepsirhines
with unfused symphyses have been
shown to recruit relatively less balancingside
than working-side adductor muscle
force during unilateral mastication (Hylander
et al., 1998, 2000). This differential
muscle recruitment results in a pattern of
lower strains along the balancing-side postorbital
bar than the working-side postorbital
bar (Ravosa et al., 2000a). Thus, an
organism with a postorbital ligament and a
stepsirhine-like adductor pattern (the latter
of which is inferred for basal primates based
on the presence of unfused symphyses; Ravosa,
1996, 1999) would experience an
asymmetrical circumorbital and, in turn, an
ocular loading pattern that is hypothesized