Memoir cover 0.tif - Ohio University College of Osteopathic Medicine

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