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WITMER-ANTORBITAL CAVITY OF ARCHOSAURS 15<br />
B<br />
-<br />
&<br />
fen antorb int<br />
prf<br />
,<br />
fen antorb<br />
lac<br />
.iua<br />
lam<br />
Jpra<br />
pmx max for neurovas<br />
FIGURE 8. Hypsilophodon foxii, facial skeleton. A, right maxilla, lacrimal,<br />
palatine, and jugal (BMNH R2477) in dorsal view. Thin arrow<br />
shows the course <strong>of</strong> the nasolacrimal duct. Thick arrow show the course<br />
<strong>of</strong> the maxillary neurovascular bundle. B, facial skeleton in left lateral<br />
view (modified after Galton, 1974.)<br />
the antorbital cavity. Here the muscle is just caudoventral to<br />
the ridge on the palatine supporting the nasal capsule and is in<br />
direct contact with the antorbital sinus (the avian homolog <strong>of</strong><br />
the caviconchal sinus <strong>of</strong> crocodilians; see below and Witmer,<br />
1995b). For much <strong>of</strong> its length, the muscle is just ventral to the<br />
suborbital diverticulum <strong>of</strong> the antorbital sinus (Witmer, 1990),<br />
and this air sac separates the pterygoideus from the other jaw<br />
muscles. As in crocodilians, the dorsal pterygoideus is ventral<br />
to the maxillary nerve (Fig. 6). In some cases (e.g., Gallus<br />
gallus), the neurovasculature closely adheres to the muscle,<br />
whereas in other cases (e.g., Anser anser) the nerve is dorsal<br />
to the muscle, suspended within the suborbital diverticulum by<br />
a double-walled, epithelial fold, a pneumatic "mesentery."<br />
Dollo (1884) reported that in birds the pterygoideus muscles<br />
attach to the rostral border <strong>of</strong> the antorbital cavity, but this is<br />
not the case in the birds I have personally dissected and appears<br />
not to have been reported otherwise in the literature. In a very<br />
few birds, a tendinous slip may contact the caudoventral tip <strong>of</strong><br />
the maxilla (Lakjer, 1926) or the palatal process <strong>of</strong> the maxilla<br />
(H<strong>of</strong>er, 1950; Burton, 1984), but this is a rare condition. Thus<br />
the dorsal pterygoideus muscle <strong>of</strong> birds is indeed <strong>of</strong>ten one <strong>of</strong><br />
the contents <strong>of</strong> the antorbital cavity, but never originates from<br />
any <strong>of</strong> the margins <strong>of</strong> the antorbital fenestrae.<br />
Osteological Correlates-The osteological correlates <strong>of</strong> M.<br />
pterygoideus, pars dorsalis are more consistent in crocodilians<br />
than in birds. In crocodilians, the medial surfaces <strong>of</strong> the jugal<br />
and maxilla tend to have a patchy striated or punctate pattern<br />
that increases somewhat in relief rostrally. The palatine usually<br />
has a raised dorsal ridge extending near the lateral edge rostral<br />
to the prefrontal articulation; this ridge marks the boundary between<br />
postconchal cartilage medially and the dorsal pterygoideus<br />
laterally. The bones surrounding the suborbital fenestra<br />
may display a slight excavation where the muscle attaches, but<br />
this fossa is usually weak and is absent in many individuals <strong>of</strong><br />
Crocodylus spp. Thus, the direct evidence in crocodilians for<br />
the presence <strong>of</strong> a dorsal pterygoideus is strongest rostrally<br />
where the muscle attaches to the maxilla, jugal, and palatine.<br />
In birds, probably because <strong>of</strong> their small size, the muscle <strong>of</strong>ten<br />
leaves little evidence on the bone. Unlike crocodilians, the avian<br />
dorsal pterygoideus originates only from the palatine and<br />
pterygoids, never from the jugal and almost never from the<br />
maxilla. The rostral attachment sometimes excavates a fossa on<br />
the dorsolateral surface <strong>of</strong> the palatine near the choana.<br />
To determine if bony surfaces could be examined directly to<br />
ascertain if muscles were attached to them, bone samples from<br />
Alligator mississippiensis and Anser anser were examined by<br />
means <strong>of</strong> scanning electron microscopy. Samples came from<br />
surfaces known to be adjacent to gland, muscle, or air sac. The<br />
results indicated that bony surfaces are highly variable in these<br />
animals, and surface textures are not reliable indicators <strong>of</strong> the<br />
adjacent s<strong>of</strong>t tissues (although surface features usually are). Although<br />
samples occasionally had the predicted surface-texture<br />
pattern (e.g., Sharpey-fiber bone for muscle [Jones and Boyde,<br />
19741 or uniformly smooth bone for gland and air sac), very<br />
<strong>of</strong>ten a sample showed the reverse pattern.<br />
Muscles are generally potent and well-understood functional<br />
matrices (sensu Moss, 1968, 1971). However, the ambiguity <strong>of</strong><br />
the direct osteological correlates in both birds and crocodilians<br />
may reflect their archosaurian heritage in that their muscles are<br />
less likely to produce reliable bony evidence (i.e., scars) than<br />
those <strong>of</strong>, for example, mammals (Bryant and Seymour, 1990,<br />
and references therein). It also reflects the small sizes <strong>of</strong> virtually<br />
all the birds and many <strong>of</strong> the crocodilians in the sample.<br />
It was found that, in the more or less complete ontogenetic<br />
series available for this study (e.g., Alligator mississippiensis,<br />
Crocodylus porosus, Struthio camelus, Anser anser, Gallus gallus),<br />
older (and hence larger) individuals had more deeply<br />
etched muscle scars.<br />
Nevertheless, despite these ambiguities, extant birds and<br />
crocodilians do have similarities that can be hypothesized to<br />
have been present in their common ancestor. For example, in<br />
both groups <strong>of</strong> extant archosaurs, the dorsal pterygoideus is a<br />
relatively large muscle originating from the dorsal or dorsolatera1<br />
surfaces <strong>of</strong> the pterygoid and palatine. The attachment on<br />
the palatine bone extends into the caudal portion <strong>of</strong> the antorbital<br />
cavity and usually excavates a fossa. This muscular fossa<br />
on the palatine is sometimes separated by a bony ridge from a<br />
fossa for the cartilaginous nasal capsule, the muscular fossa<br />
being generally caudoventrolateral to the nasal fossa. In neither<br />
group does the muscle fill the entire antorbital cavity, but rather<br />
is restricted to the caudal portion <strong>of</strong> the cavity, behind a homologous<br />
air sac (the crocodilian caviconchal sinus and avian<br />
antorbital sinus; see below and Witmer, 199513).<br />
Direct information regarding the extent and position <strong>of</strong> the<br />
muscle can be obtained by making use <strong>of</strong> the fact that in both<br />
birds and crocodilians (indeed in all sauropsids) the maxillary<br />
nerve always travels dorsal to the pterygoideus musculature<br />
(Fig. 6A, B). Thus, the position <strong>of</strong> the foramen or groove for