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WITMER-ANTORBITAL CAVITY OF ARCHOSAURS
1961; Bakker, 1986; Paul, 1988a) and the presence of just such
a muscle in crocodilians (Adams, 19 19; Anderson, 1936).
Again, paraphyletic treatment of archosaurs (i.e., excluding
birds) has compromised the resulting interpretations.
Implications of the Muscular Hypothesis
A muscle-related antorbital fenestra and fossa is such a pervasive
notion that it is worthwhile to examine its implications
briefly before testing the hypothesis with the EPB approach.
For example, the fact that a nasal cavity must both exist and
function seems to be overlooked in some formulations of the
muscular hypothesis. If a muscle completely filled the antorbital
cavity and fossa of a narrow-snouted archosaur such as Coelophysis
bauri (Bakker, 1986) or Postosuchus kirkpatricki, then
there would be simply no room to accommodate the nasal cavity
and capsular structures such as the nasal conchae. Parasuchians
present a striking example. Camp (1930; see also Anderson,
1936) suggested that the median cavity within the premaxillae
rostral to the nares was filled with muscle in parasuchians.
If this were the case, the contralateral muscles together
would have formed a sling-like sphincter, constricting the nasal
capsule with each contraction-certainly an unlikely arrangement.
Furthermore, in many formulations, such as Janensch's
(1935-36), the muscle would have to pass over, and thus occlude,
the choana.
Another problem involves the architecture of the bones forming
the internal antorbital fenestrae. As mentioned, the superficial
appearance sometimes resembles that of known muscular
fossae such as that surrounding the laterotemporal fenestra.
However, the two fossae differ in detail in that the laterotemporal
fenestra has rounded, heavily buttressed edges whereas
the internal antorbital fenestra often has thin, delicate, sharp
edges. A muscle passing through the internal antorbital fenestra
could attach to the maxillary antorbital fossa with little apparent
problem, but would have to curve around the rostral border of
the lacrimal and pass caudally to fill the lacrimal antorbital fossa.
However, the rostral border of the lacrimal is often bladelike
(e.g., Ornithosuchus longidens, BMNH R3 143) and sometimes
paper-thin (e.g., Lesothosaurus diagnosticus, BMNH
R11956, RUB17, R8501; see Fig. 7), and does not seem competent
to resist muscular stresses.
A different course for the muscle was suggested by Galton
(1974) for Hypsilophodon foxii, passing not through the internal
antorbital fenestra but rather through an opening interpreted
here as a neurovascular canal (see below and Fig. 8). Galton
(1974) also reconstructed a portion of the adductor musculature
(the ventral pterygoideus) as passing through the suborbital fenestra
to attach within the antorbital cavity, but the suborbital
fenestra does not transmit muscle in this manner in any sauropsids,
so this idea can be safely discounted.
Some archosaurs (e.g., some large pterodactyloid pterosaurs
and some theropods) have cavities and chambers associated
with their lacrimal and/or maxillary antorbital fossae. The septa
within these chambers seem ill-equipped to withstand the forces
of muscular contraction. In fact, the entire structure of the snout
of some archosaurs seems too frail to withstand such loads. For
example, Bakker (1986: 262) regarded the enormous antorbital
fenestrae of Coelophysis bauri and the pterosaur Dimorphodon
macronyx as filled with an equally enormous muscle. Considering
the thin bars of bone of which these skulls are constructed,
such a muscular system probably would not have had the
opportunity to contract more than once!
As alluded to earlier, the impetus for the notion of a muscular
antorbital cavity historically has come more from theory than
empirics, invoking the "need" for a large muscle originating
on the snout. Walker (1961) articulated this argument most
clearly, and it has been summarized previously (Witmer,
1987b). Bakker (1986) and Paul (1988a) also believed a large
antorbital muscle was necessary for rapid adduction of the mandible,
"snapping" the jaws shut. Many of Ewer's (1965) criticisms
of Walker's formulation are on target, but the point here
is that even if formal biomechanical analysis predicts a large
muscle, that prediction alone is insufficient to reconstruct the
muscle within the antorbital fenestra and fossa.
The Extant Phylogenetic Bracket
Only a single candidate for the "antorbital muscle" has been
proposed: a rostral portion of M. pterygoideus, in particular, M.
pterygoideus, pars dorsalis (or simply "dorsal pterygoideus").
The muscle has had several different designations over the
years (e.g., M. adductor mandibulae internus pterygoideus anterior,
M. pterygoideus internus, M. pterygoideus anterior, pterygoideus
D), but its homology among sauropsids is generally
unquestioned (Adams, 1919; Lakjer, 1926; Lubosch, 1933;
Edgeworth, 1935; Kesteven, 1945; see also Witmer, 1995b).
The precise hierarchical level within Sauropsida at which division
into dorsal and ventral portions of the muscle occurred
is unclear, yet all workers agree that extant birds and crocodilians
have a homologous dorsal pterygoideus muscle (Witmer,
1995b). As will be seen, the maxillary division of the trigeminal
nerve figures into the argument, and its homology across Vertebrata
also is unquestioned (Witmer, 1995b). In extant archosaurs,
the size of the nerve varies greatly, being reduced in most
neornithine birds in association with reduction of the maxillary
bone and loss of the teeth (Witmer, 199%) but remaining large
in crocodilians. The maxillary nerve carries general somatic
afferent fibers (as well as postganglionic autonomic fibers from
the sphenopalatine ganglion; Bubien-Waluszewska, 1981) and
is not to be confused with the pterygoideus nerves, which are
motor branches of the mandibular division of the trigeminal
nerve.
Extant Crocodilians-The adductor muscles of extant crocodilians
have been studied extensively (Schumacher, 1973; Busbey,
1989). The rostral attachments of the muscle are briefly
described below (Fig. 6A), based mostly on original dissections
(for details see Witmer, 1995b). The dorsal pterygoideus is a
very large muscle passing dorsally over the palatal bones, ventral
to the eyeball, and through the postnasal fenestra to fill the
caudolateral portion of the antorbital cavity. It attaches to or is
in contact with the pterygoid, ectopterygoid, jugal, maxilla, palatine,
prefrontal, lacrimal, interorbital septum, and the caudolateral
surface of the postconcha (a portion of the cartilaginous
nasal capsule). Rostrally, the muscle tapers to a point where it
attaches to the maxilla just lateral to the ostium of the caviconchal
paranasal air sinus. The maxillary nerve and accompanying
vessels travel through the orbit over the dorsal surface of
the muscle (Fig. 6A), passing to the muscle's rostral tip where
they enter a large foramen within the maxilla just lateral to the
caviconchal sinus ostium (see Witmer, 1995b). Thus, in extant
crocodilians, M. pterygoideus, pars dorsalis is indeed one of
the contents of the antorbital cavity and, in fact, fills the caudolateral
portion of the cavity.
Extant Birds-As in crocodilians, the dorsal pterygoideus
of birds is usually a large muscle, although its size varies greatly
(see Witmer, 1995b and references therein for variations). In
general, the muscle originates from the dorsolateral surfaces of
the palatine and pterygoid bones (Fig. 6B). Since the ectopterygoid
bone and transverse pterygoid flange have been lost in
birds (at least above the phylogenetic level of Archaeopteryx
lithographica; Witmer and Martin, 1987; Elzanowski and Wellnhofer,
1996), the muscle fibers extend in a relatively straight
line caudoventrally to the mandible, rather than curving around
the palatal bones as they do in crocodilians. The rostral attachment
on the palatine often reaches the caudoventral portion of