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

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142 YEARBOOK OF PHYSICAL ANTHROPOLOGY [Vol. 43, 2000 potentially useful analytical concept for researchers interested in integration between the cranial base and face because it effectively characterizes both the posterior margin of the face and the boundary between the anterior and middle cranial fossae in lateral radiographs. The inferior terminus, Ptm, is the posterolateral corner of the ethmomaxillary complex and lies just in front of the spheno-palatine suture (Williams et al., 1995). The superior terminus, PMp, is the anterior-most point of the middle cranial fossa, lying close to the midpoint of the spheno-ethmoid synchondrosis and the midpoint of the spheno-frontal suture on the floor of the cranial base in all primates (Van der Linden and Enlow, 1971; McCarthy, 2001; McCarthy and Lieberman, 2001). Perhaps the most interesting aspect of the PM plane is its relationship to the orbits and the anterior cranial base. Several researchers have claimed that the PM plane always forms a 90° angle to the neutral horizontal axis (NHA) of the orbits (see Measurement Definitions). In their initial study, Enlow and Azuma (1975) found the PM- NHA angle to average 90° in a combined mammalian sample of 45 species, and 90° in a large sample of adult humans. Ravosa (1991a,b), and Ravosa and Shea (1994) tested the PM-NHA angle in a cross-sectional sample of macaques and two interspecific sample of adult primates, and obtained consistent, but different PM-NHA angles from those of Enlow and Azuma (1975), that ranged between 18° and 5° below 90°. However, these studies measured the PM plane and the NHA (the latter only slightly) differently, and several more recent studies have corroborated the original hypothesis of Enlow and Azuma (1975). In particular, Bromage (1992) found the PM- NHA angle in a cross-sectional sample of 45 Pan troglodytes crania to be 89.2 3.4° SD for dental stage I, 90.5 3.1° SD for dental stage II, and 88.2 4.0° SD for dental stage III. However, these data show some significant variation during growth, and some adult crania have PM-NHA angles somewhat different from 90°, especially those for certain hominids. Lieberman (1998) found the PM-NHA angle to be 89.9 1.7° SD in a longitudinal series of humans (Denver Fig. 11. Histograms comparing mean PM-NHA angle in samples of 18 adult anthropoid species (top) and 15 adult strepsirrhines species (bottom). PM-NHA° is not significantly different from 90° in any species. Growth Study; n 353) aged 1 month through 17 years, 9 months. Also, McCarthy and Lieberman (in press) recently found the PM-NHA angle to average 90.0 0.38° SD in a pooled sample of adults from 18 anthropoid species, and 89.4 0.46° SD in a pooled sample of adults from 15 strepsirhine species (Fig. 11). Consequently, the PM-NHA does appear to be invariant in primates, with values for the most part near 90°. It should be stressed, however, that the developmental and functional bases (if any) for this purported invariance are still unknown and require further study. The 90° PM-NHA angle is useful for examining craniofacial integration and variation because, as noted above, the NHA is tightly linked to the orientation of the anterior cranial fossa and the ethmomaxillary complex. The roofs of the orbits (which help
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,
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