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16 SOCIETY OF VERTEBRATE PALEONTOLOGY. MEMOIR 3
the maxillary nerve (or neurovascular bundle, since vessels accompany
the nerve) provides a sensitive guide to the maximal
dorsal extent of the muscle. In crocodilians, the dorsal pterygoideus
fills the caudal part of the antorbital cavity, and the
foramen is dorsally situated. In birds, the dorsal pterygoideus
is restricted to the palatine bone at the caudoventral corner of
the antorbital cavity, and the maxillary nerve traverses a foramen
or gap between maxilla and palatine. Despite the difference
in size and apparent position of the muscle, the position
of the maxillary neurovascular foramen faithfully indicates the
muscle's general location.
The Hypothesis
Given these correspondences between the components of the
extant phylogenetic bracket, we may hypothesize that the common
ancestor of Archosauria had a large M. pterygoideus, pars
dorsalis that had the following characteristics: (1) it originated
from the palatine and pterygoid and probably excavated a fossa
on the palatine, and (2) it was situated ventral to the maxillary
neurovascular foramen andlor grooves, indicating that the muscle
was restricted to the caudoventral portion of the antorbital
cavity. This hypothesis is tested by searching for the osteological
correlates in the other, extinct descendants of the common
ancestor.
Testing the Hypothesis
There is abundant evidence in most major clades of fossil
archosaurs for the presence and general position of a dorsal
pterygoideus muscle. As will become apparent, most fossil archosaurs
resemble extant birds more than crocodilians. As a
result, crocodylomorphs deserve special attention.
Crocodylomorpha-In many fossil crocodylomorphs, the
prefrontal bone has a transversely broad flange that projects far
ventrally into the postnasal fenestra (Fig. 1) diverting any musculature
ventrally and creating a cavity rostral to it within the
nasoantorbital cavity. This prefrontal flange is found in Dibothrosuchus
elaphros (IVPP V 7907; Wu and Chatterjee, 1993),
Sphenosuchus acutus (Walker, 1990), Protosuchus richardsoni
(UCMP 130860; Clark, 1986), all thalattosuchians examined for
this study, and Theriosuchus pusillus (BMNH 48330), among
others, and is probably primiti,ve for Crocodylomorpha if not a
more inclusive group (it is somewhat developed in the stagonolepidids
Desmatosuchus haplocerus [TTUP 90231 and Stagonolepis
robertsoni [Walker, 19611, but apparently not in Postosuchus
kirkpatricki [TTUP 9000, 90021). In many of these (e.g.,
Protosuchus richardsoni, Pelagosaurus typus [BMNH 325991,
Metriorhynchus superciliosus [BMNH R39001, Theriosuchus
pusillus), the prefrontal flange is a delicate, thin plate that appears
too fragile to serve as area for adductor muscle attachment;
rather, it probably supported the nasal capsule. Thus, the
muscle was probably displaced ventrally relative to extant crocodilians.
The suborbital fenestra also provides some measure of the
rostral extent of the muscle. In basal crocodylomorphs (sphenosuchians
and protosuchians), the suborbital fenestra, at most,
barely reaches into the antorbital cavity. In Sphenosuchus acutus,
there is a distinct muscular fossa on the dorsal surface of
the palatine, just rostral to the suborbital fenestra (Walker, 1990:
fig. 3b); the area rostral to this crest is probably associated with
the nasal cavity in some way. In most mesoeucrocodilians,
however, the suborbital fenestra-and presumably the dorsal
pterygoideus-is canied farther into the antorbital cavity. In
some thalattosuchians (e.g., Metriorhynchus superciliosus,
BMNH R2048), there is a tapering fossa or groove on the dorsal
surface of the palatine rostral to the suborbital fenestra, whereas
in others (e.g., Pelagosaurus typus) there is little direct evidence
of the muscle.
With regard to the maxillary neurovasculature, Walker (1990)
described and figured for Sphenosuchus acutus a large medial
foramen and groove in the body of the maxilla just internal to
the antorbital fenestra and dorsal to the palatine, suggesting
that, as in birds, the muscle was restricted to the caudoventral
aspect of the antorbital cavity. Similarly, in the protosuchian
Shantungosuchus hangjinensis (Wu, Brinkman, and Lii, 1994)
and the thalattosuchians Pelagosaurus typus (BMNH 32599,
32607) and Metriorhynchus superciliosus (BMNH R3900), the
maxillary neurovascular foramina enter the medial surface of
the bone ventral to the antorbital fenestra. In Sebecus icaeorhinus
(AMNH 3160), the neurovascular foramina also enter the
maxilla internally just dorsal to the teeth, suggesting limited
dorsal extent of the dorsal pterygoideus, which is in agreement
with Colbert's (1946a) reconstruction of that muscle.
Thus, basally in crocodylomorphs, there is good evidence
that the dorsal pterygoideus did not extend beyond the caudoventral
portion of the antorbital cavity. Although this study has
not sought to determine at which level in Crocodylomorpha the
modern condition appeared, it may coincide with loss or transformation
of the descending transverse flange of the prefrontal.
Alternatively, it may coincide with the appearance of the morphogenetic
rotation of the nasal cavity (Witmer, 1995b) that, in
extant crocodilians, brings the neurovasculature to a dorsal position
in the snout relative to its primitive, more ventral position.
Other Crurotarsi-There appear to be two types of putative
muscular fossae on the dorsal surfaces of the palatine bones in
other crurotarsan archosaurs. In the first type (found also in the
crocodylomorph Sphenosuchus acutus [Walker, 1990]), there is
a well defined fossa just rostral to and clearly associated with
the suborbital fenestra. Stagonolepis robertsoni has this type of
fossa, clearly possessing (BMNH R8582) a dorsal excavation
of the palatine rostral to the suborbital fenestra that strongly
undercuts the rostral portion of the palatine and curves onto the
pterygoid and maxilla (Fig. 9C). The rostral portion of the palatine
also has a fossa that must be associated with the nasal
cavity in some way. It floors the antorbital cavity caudal to the
choana in S. robertsoni, but in Desmatosuchus haplocerus
(TTUP 9023) this rostral portion of the palatine appears to extend
rostrally lateral to the choana to attach to the ascending
rarnus of the maxilla, forming a partial rostromedial wall to the
antorbital cavity. Furthermore, in Longosuchus meadei, processes
from the lacrimal and prefrontal essentially close the
postnasal fenestra, preventing any substantial muscular incursion
into the antorbital cavity (Parrish, 1994). Thus, stagonolepidids
would seem to have had a caudoventrally restricted
dorsal pterygoideus.
In the second type of muscular fossa on the palatine, the
fossa appears to extend rostrally all the way up to the choana,
resembling the situation in birds. This condition characterizes
many groups of archosaurs and may well be the primitive condition
although the situation in many nonarchosaurian archosauriforms
is unknown and is equivocal for many archosaurs.
Ornithosuchus longidens (BMNH R3143; see also Walker,
1964) has this type of fossa, displaying a large excavation on
the dorsal surfaces af the palatine and pterygoid bones extending
rostrally up to and undercutting a strong, elevated ridge or
strut bordering the choana (Fig. 10C). This postchoanal strut
itself is grooved, presumably for attachment of'the cartilaginous
nasal ca~sule. Parasuchians also show this condition. usuallv
with the maxilla also contributing to the floor of the antorbital
cavity. In most parasuchians (e.g., Pseudopalatus pristinus,
AMNH 7222; well figured by Case [1929: fig. 161 and Camp
[1930: fig. 33]), the palatine and vomer are drawn up dorsally
behind the choana into a spout-like structure directed toward
the naris. The conjoined transverse postchoanal crests of the
vomer and palatine often trend caudolaterally, delimiting a fos-