The primate cranial base: ontogeny, function and - Harvard University
The primate cranial base: ontogeny, function and - Harvard University
The primate cranial base: ontogeny, function and - Harvard University
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D.E. Lieberman et al.]<br />
PRIMATE CRANIAL BASE 153<br />
in relative encephalization by expansion of<br />
the brain forward over the top of the orbits.<br />
It has been argued that, as predicted by<br />
Dabelow (1931), by bringing the orbits <strong>and</strong><br />
anterior <strong>cranial</strong> <strong>base</strong> into close continuity,<br />
they became structurally integrated such<br />
that changes in the orientation of one necessarily<br />
affect the other (Ross <strong>and</strong> Ravosa,<br />
1993). Small nocturnal strepsirhines, in<br />
contrast, tend to converge their orbits towards<br />
the top of their head, not compressing<br />
the olfactory apparatus, <strong>and</strong> preventing<br />
brain expansion over the orbits concomitant<br />
with a shift to diurnality. <strong>The</strong> orbits <strong>and</strong><br />
anterior <strong>cranial</strong> <strong>base</strong> in all strepsirhines are<br />
not as unified structurally, <strong>and</strong> thus basi<strong>cranial</strong><br />
flexion <strong>and</strong> orbit orientation do not<br />
covary as greatly.<br />
<strong>The</strong> evidence that all omomyiforms that<br />
are well enough preserved have an interorbital<br />
septum below the olfactory tract, like<br />
that of Tarsius <strong>and</strong> small anthropoids (Ross,<br />
1994), suggests that this integration of anterior<br />
<strong>cranial</strong> <strong>base</strong> <strong>and</strong> upper face may be an<br />
early unique derived feature of the haplorhine<br />
stem lineage. This integration set the<br />
stage for the subsequent evolution of the<br />
haplorhine postorbital septum. Increased<br />
orbital frontation (klinorhynchy) <strong>and</strong> convergence<br />
in early anthropoids caused the<br />
anterior temporal muscles <strong>and</strong> orbit to come<br />
into close proximity, leading to the evolution<br />
of a postorbital septum (Ross, 1995b, 1996).<br />
Exactly why increased frontation occurred<br />
in haplorhines remains unexplained, although<br />
it has been argued that increased<br />
relative brain size might have caused increased<br />
basi<strong>cranial</strong> flexion, producing increased<br />
orbital frontation as a consequence<br />
of the integration of the anterior <strong>cranial</strong><br />
<strong>base</strong> <strong>and</strong> orbits (Ross, 1996; see also Ravosa<br />
et al., 2000a on the effects of small size on<br />
skull form in basal <strong>primate</strong>s). This hypothesis<br />
remains to be evaluated in fossil stem<br />
anthropoids for want of well-preserved specimens,<br />
but is not supported by the fact that<br />
early anthropoids had a postorbital septum<br />
in conjunction with relatively small brains<br />
(Ross, 2000). It remains to be determined<br />
why the haplorhine stem lineage evolved<br />
more frontated orbits, but the findings of<br />
Strait <strong>and</strong> Ross (1999) suggest that the role<br />
of posture should be considered.<br />
Variation in hominin <strong>cranial</strong> <strong>base</strong> angle<br />
How to account for variation in basi<strong>cranial</strong><br />
flexion in hominins has been controversial<br />
<strong>and</strong> remains unresolved. Several recent<br />
studies (e.g., Ross <strong>and</strong> Henneberg, 1995;<br />
Spoor, 1997) have shown that the <strong>cranial</strong><br />
<strong>base</strong> in Australopithecus <strong>and</strong> Homo is generally<br />
more flexed than in Pan or other nonhuman<br />
<strong>primate</strong>s. This difference raises two<br />
questions. First, how much is <strong>cranial</strong> <strong>base</strong><br />
flexion among early hominins related to upright<br />
posture, facial orientation, or brain<br />
size? Second, what factors account for the<br />
observed variation in <strong>cranial</strong> <strong>base</strong> angle<br />
among hominins?<br />
Although <strong>cranial</strong> <strong>base</strong> angles vary substantially<br />
among hominin species, australopithecines<br />
have CBAs generally intermediate<br />
between humans <strong>and</strong> chimpanzees, but<br />
with more flexed <strong>cranial</strong> <strong>base</strong>s among A.<br />
boisei <strong>and</strong> less flexed <strong>cranial</strong> <strong>base</strong>s for A.<br />
aethiopicus (KNM-WT 17000) (F. Spoor,<br />
personal communication). Until now, the<br />
hypothesis that <strong>cranial</strong> <strong>base</strong> flexion in hominins<br />
is an adaptation for increased brain<br />
size relative to basi<strong>cranial</strong> length (Gould,<br />
1977) has received the most support (Ross<br />
<strong>and</strong> Ravosa, 1993; Ross <strong>and</strong> Henneberg,<br />
1995; Spoor, 1997; McCarthy, 2001). In particular,<br />
if one measures IRE5 vs. CBA1<br />
(thereby including the cribriform plate in<br />
measures of both basi<strong>cranial</strong> length <strong>and</strong> the<br />
angle of the <strong>cranial</strong> <strong>base</strong>), then H. sapiens<br />
<strong>and</strong> other hominin taxa such as A. africanus<br />
have exactly the degree of flexion expected<br />
by basi<strong>cranial</strong> length.<br />
However, not all of the variation in hominin<br />
CBA can be explained by relative brain<br />
size. For example, Ne<strong>and</strong>erthals <strong>and</strong> archaic<br />
Homo fossils such as Kabwe have considerably<br />
more extended CBA1s than H. sapiens<br />
(about 15°), even though they are<br />
bipedal <strong>and</strong> similarly encephalized (Ruff et<br />
al., 1997). In addition, other measures of<br />
<strong>cranial</strong> <strong>base</strong> angle <strong>and</strong> relative encephalization,<br />
which do not include the cribriform<br />
plate (CBA4 <strong>and</strong> IRE5), indicate that H.<br />
sapiens have less flexed <strong>cranial</strong> <strong>base</strong>s then<br />
expected for anthropoids of their size (Ross<br />
<strong>and</strong> Henneberg, 1995). This finding highlights<br />
the likelihood that no single explanation<br />
will account for interspecific differences