Facial Torsion and the Evolution of the Primate Postorbital

Based on the galago strain-magnitude data, it appears that the primate postorbital bar is overbuilt to resist masticatory stresses. Thus, there is more than enough cortical bone to provide rigid lateral orbital margins. Perhaps once ossified (for whatever nonmasticatory function), the postorbital bar must

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Figure 4. Average directions of for the left postorbital bar during mastication on the working (L) and balancing (R) side (adapted from Ravosa et al., 2000b) (Table 2). The line "A" marks the orientation of the A element of the delta-rosette gage in each of six experiments. The grand mean (±49°) is close to the predictions of the facial torsion model; however, this is opposite what should be found in galagos. Only WS postorbital bar stain patterns are as predicted by the NVPH.

then be constructed of an amount of tissue sufficient to counter accidental, traumatic loads to its outwardly flared margins (Hylander and Johnson, 1992, 1997a,b; Hylander and Ravosa, 1992; Hylander et al., 1991a,b; Ravosa et al., 2000a,b). As argued earlier, circumorbital structures are probably not specially designed to counter masticatory stresses because bone in this area could be reduced considerably without risking structural failure associated with forceful chewing and biting (Bouvier and Hylander, 1996a,b; Hylander and Johnson, 1992, 1997; Hylander and Ravosa, 1992; Hylander et al., 1991a,b; Ravosa et al., 2000a,b,d; Ross and Hylander, 1996). To do so, however, increases the risk of fracturing the circumorbital region due to accidental, traumatic external forces of a nonmasticatory nature (e.g., excessive loads during falls), and this would significantly reduce the fitness of such an organism (Hylander and Johnson, 1992, 1997a,b; Hylander and Ravosa, 1992;

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Figure 5. Average directions of £1 during left-sided (L) and right-sided (R) mastication along the dorsal interorbit of greater galagos (adapted from Ravosa et al., 2000b) (Table 2). The line marked "A" represents the orientation of the A element of the delta-rosette strain gage during each of the three experiments. The direction of peak maximum principal strain (£1) is always given relative to the A element. The grand mean of ±56° is close to predictions of the facial torsion model, although this may be opposite what should be observed in galagos (see text).

Figure 5. Average directions of £1 during left-sided (L) and right-sided (R) mastication along the dorsal interorbit of greater galagos (adapted from Ravosa et al., 2000b) (Table 2). The line marked "A" represents the orientation of the A element of the delta-rosette strain gage during each of the three experiments. The direction of peak maximum principal strain (£1) is always given relative to the A element. The grand mean of ±56° is close to predictions of the facial torsion model, although this may be opposite what should be observed in galagos (see text).

Hylander et al., 1991a,b; Ravosa et al., 2000a,b,d). Therefore, selection for circumorbital and neurocranial safety factors of higher magnitude than those for masticatory elements is especially critical for any skeletal structure that houses and/or protects special sense organs.

If the evolution of a novel circumorbital structure such as the postorbital bar has been a two-step process, this has direct implications for the facial torsion model. For instance, it could be argued that the postorbital bar of the first primates was initially an adaptation to resist facial twisting during mastication and that in subsequent basal taxa, the amount of cortical bone along the lateral orbital margin was increased so as to ensure an adequate safety factor for accidental nonmasticatory forces. That is, once of adequate size (to counter traumatic loads), the postorbital bar of modern primates now experiences negligible strains during postcanine biting and chewing.3

Nevertheless, several lines of evidence directly refute the importance of the facial torsion model. First, and perhaps most importantly, neither the postorbital bars nor browridges of primates are oriented 45° relative to the cranial long axis (Hylander and Ravosa, 1992; Ravosa, 1991a,b; Ravosa et al., 2000a,b). Second, our analysis of jaw-adductor activity patterns and condylar reaction forces indicates that, while the galago circumorbital region is likely twisted, the underlying causative forces differ from the predictions of the facial torsion model. Finally, circumorbital strain directions for three anthropoids also contradict the facial torsion model (Hylander et al., 1991a,b; Ross and Hylander, 1996)—a finding which undermines the applicability of this model to primates.

Interestingly, the galago strain-direction pattern contrasts with that for the anthropoid dorsal interorbital region. Whereas papionins and owl monkeys also show a reversal pattern, interorbital strain directions are opposite those for galagos and thus, contrary to predictions of the facial torsion model (Hylander and Johnson, 1992; Hylander and Ravosa, 1992; Hylander et al., 1991a,b; Ross and Hylander, 1996). There are two interrelated causes of this apparent suborder difference in circumorbital principal-strain directions. In contrast to anthropoids (and the facial torsion model), condylar reaction forces and jaw-adductor forces during mastication are largely localized to the working side of the galago face (and presumably that of all strepsirhines and "prosimians" with unfused symphyses - Hylander, 1979a,b; Hylander et al., 1998, 2000, 2004; Ravosa and Hylander, 1994; Ravosa and Hogue, 2004; Ravosa et al., 2000a; Vinyard et al., this volume). This is reflected in the large disparity in WS/BS peak-strain magnitude ratios for bilateral structures such as the postorbital bar and mandibular corpus (Table 1). On the other hand, anthropoids recruit relatively higher BS jaw-adductor forces and, in turn, experience less variation in cranial peak strains between working and balancing sides (Hylander, 1979a; Hylander et al., 1991a,b, 1998; Ross and Hylander, 1996). Therefore, despite the fact that primate circumorbital structures are not designed to resist masticatory stress, it is now evident that suborder differences in skull form

3 Our explanation differs from the argument that a bony postorbital bar serves to protect the lateral aspect of the eye from injury when a greater proportion of the lateral orbital margin lies exposed to branches during locomotion (Prince, 1953; Simons, 1962). This latter suggestion is flawed because an ossified postorbital bar would characterize a significantly greater number of mammalian clades if its function were solely protective (Cartmill, 1970, 1972).

significantly influence suborder patterns of stress along the circumorbital region. Indeed, as galago forces during mastication differ from expectations or assumptions of the facial torsion model, but nonetheless result in circumorbital strain directions in support of this model; it is unlikely that the strepsirhine (Ravosa et al., 2000a,b) or anthropoid (Hylander et al., 1991a,b) skull functions as a simple hollow cylinder. This highlights the considerable benefit of modeling complex biological systems with a broad, phylogenetic characterization of in vivo patterns of functional variation (Hylander et al., 1998, 2000, 2004; Lauder, 1995; Ravosa et al., 2000a,b).

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