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

D.E. Lieberman et al.]
PRIMATE CRANIAL BASE 143
define the NHA) comprise much of the floor
of the anterior cranial fossa. Therefore, it
follows that the PM plane and the anterior
cranial base should also form an approximately
90° angle in primates whose orbits
are approximated to the midline. This hypothesis
was tested by McCarthy and
Lieberman (2001), who found that the angle
between the PM plane and the planum
sphenoideum averaged 95.2 7.6° SD (n
18) in anthropoids and 82.8 9.5° SD (n
14) in strepsirhines. McCarthy and Lieberman
(2001) also found that the angle between
the PM plane and the midline anterior
cranial base (from the sella to the
foramen caecum) averages 89.2 9.97° SD
(n 18) in anthropoids, but 70.6 10.5° SD
(n 15) in strepsirrhines. The high standard
deviations of these angles indicate that
the integration between the back of the face
and the anterior cranial base is not very
strong. Ross and Ravosa (1993) also came to
similar conclusions by comparing the orbital
axis orientation relative to the posterior cranial
base, against the orientation of the planum
sphenoideum relative to the posterior
cranial base. The differences between anthropoids
and strepsirhines in the relationship
of the orbits to the cranial base can be
explained by the fact that the roof of the
orbits does not contribute to the midline
cranial base in strepsirhines, and because
the cribriform plate tends to be oriented
more vertically relative to the planum sphenoideum
in strepsirrhines than in anthropoids
(Cartmill, 1970).
The potential integration of the middle
and anterior cranial fossae with the face (as
measured via the PM plane) and the anterior
cranial base raises several interesting
issues. Most importantly, the top and back
of the face appear to form an integrated
unit, the “facial block” which rotates during
ontogeny around an axis through the intersection
of the anterior and middle cranial
fossae at the front of the greater wings of
the sphenoid (McCarthy and Lieberman,
2001). This facial block is characteristic of
anthropoids but not strepsirhines, and manifests
itself through correlations between
cranial base angle and upper facial orientation
in primates (Weidenriech, 1941; Moss
and Young, 1960; Biegert, 1963; Shea,
1985a, 1986, 1988; Ravosa, 1988, 1991a,b;
Ross and Ravosa, 1993; Ross, 1995a,b; May
and Sheffer, 1999; Lieberman, 2000; Ravosa
et al., 2000a, 2000b). In particular, as the
anterior cranial base flexes relative to the
posterior cranial base, the PM plane also
must flex relative to the posterior cranial
base, rotating the posterior and upper portions
of the face underneath the anterior
cranial fossa (klinorhynchy). In contrast, extension
of the anterior cranial base relative
to the posterior cranial base will rotate the
posterior and upper portions of the face dorsally
relative to the posterior cranial base
(airorhynchy) (Fig. 12).
The relationship of the orientation of the
back of the face (as measured for example by
the PM plane) to the anterior cranial base
also influences nasopharynx shape. As Figure
12 shows, flexion of the anterior cranial
base and/or face relative to the posterior
cranial base not only rotates the face under
the anterior cranial fossa, but it also shortens
(absolutely and relatively) the length of
the pharyngeal space between the back of
the palate and the front of the vertebral
column (Laitman and Heimbuch, 1982;
Spoor et al., 1999; McCarthy and Lieberman,
2001). While flexion of the cranial base
during ontogeny is completely independent
of the descent of the hyoid and larynx
(Lieberman and McCarthy, 1999), variation
in cranial base angle does influence some
aspects of pharyngeal shape (Laitman and
Heimbuch, 1982; see below).
Ross and Henneberg (1995) suggested
that there must be functional constraints on
how far back the hard palate can be positioned
without occluding the airway. The
integration of the anterior cranial base with
the upper and posterior margins of the face
means that these constraints on pharynx
position might determine the maximum
possible degree of basicranial angle, particularly
in genera such as Pongo and Alouatta
with relatively large pharyngeal structures
(Biegert, 1957, 1963). Ross and Henneberg
(1995) suggested that hominoids might
have found a way to circumvent these “constraints.”
Hominoids have more airorhynch
(dorsally rotated and less frontated) orbits
and palates than nonhominoid primates
with comparably flexed basicrania (Shea,