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SOCIETY OF VERTEBRATE PALEONTOLOGY, MEMOIR 3
lac
max antorb antorb jug
int lac
FIGURE 27. Metriorhynchus superciliosus. Reconstruction of antorbital
region in left lateral view based on BMNH R3900, R4762, and
R3899.
R3353). Thus, although requiring further confirmation, it is possible
that absence of the nasolacrimal canal (and presumably
the duct, as well) may characterize this already distinctive
group.
In extant crocodilians, the antorbital cavity is located completely
internally. Pelagosaurus typus also appears to have "internalized"
its antorbital cavity. In P. typus (BMNH 32599,
32607), a conical paranasal chamber within the maxilla is separated
from the nasal cavity proper by a thin, caudally emarginate
septum (Fig. 26B). This chamber is immediately rostra1
to the external antorbital fenestra and clearly associated with
the antorbital cavity in that the dorsolateral attachment of the
septum runs caudodorsally over the fenestra, thus fonning its
dorsal margin. The maxillary neurovasculature also traversed
the paranasal chamber, as indicated by foramina in the lateral
wall of the chamber, but the chamber appears too large solely
for vessels and nerves. The caudal aperture of this paranasal
chamber closely resembles the aperture leading into the caviconcha1
sinus in modem crocodilians. Thus, the caudal margin
of the septum is probably equivalent to the internal antorbital
fenestra of other archosaurs, which implies that the chamber is
the antorbital cavity.
Metriorhynchus superciliosus (BMNH R3900, R2048) clearly
lacks such an internal paranasal chamber but retains a moderately
large external antorbital fossa. Thus, it seems likely that,
in Pelagosaurus typus, the formerly external cavity has been
enclosed laterally and internalized, constricting the external fenestra
to a slit. In both P. typus and metriorhynchids, a groove
extends rostrally from the external fenestra, which is consistent
with this assessment. Antunes (1967) suggested that this groove
conducted neurovasculature, which is a reasonable interpretation.
If the situation in P. typus is interpreted correctly here, its
internalization of the antorbital cavity would represent an acquisition
independent of that observed in extant crocodilians,
because several higher mesoeucrocodilians display the primitive
condition (i.e., a relatively large external antorbital fenestra,
well developed external antorbital fossa).
Higher mesoeucrocodilians (i.e., Metasuchia; Benton and
Clark, 1988; Wu, Li, and Li, 1994) include a series of forms
that exhibit aspects of facial morphology more similar to basal
crocodylomorphs than to thalattosuchians. These include Hsisosuchus
chungkingensis (Li et al., 1994; although its precise
phylogenetic position is somewhat uncertain: see Wu, Li, and
ch sec
FIGURE 28. Araripesuchus gomesii. A, Drawing of snout in left lateral
view (modified after Hecht, 1991). B, same in ventral view (modified
after Gasparini, 1971).
Li, 1994), Notosuchus terrestris (Woodward, 1896; Gasparini,
1971; Bonaparte, 1991b), a notosuchid from Malawi (Clark et
al., 1989), Uruguaysuchus aznarezi (Rusconi, 1932; Gasparini,
1971), and Araripesuchus gomesii (AMNH 24450; Price, 1959;
Gasparini, 197 1; Hecht, 199 1; Fig. 28). All have a small internal
antorbital fenestra between the maxilla and lacrimal that
leads medially into the nasal cavity (the position of the primary
choana has not been described). Similarly, all these forms exhibit
a well-developed antorbital cavity excavating a fossa on
the maxilla and lacrimal. In at least Notosuchus terrestris and
Uruguaysuchus aznarezi, the antorbital cavity extends deeply
rostroventrally into the maxilla beyond the margin of the external
antorbital fenestra (Rusconi, 1932; Gasparini, 1971 ; Bonaparte,
199 1 b), perhaps forming an accessory cavity. According
to Gasparini (1971), the nasolacrimal canal in N. terrestris, U.
aznarezi, and A. gomesii passes through the lacrimal to open
within the caudodorsal portion of the antorbital cavity; from
here the duct presumably passed medially through the internal
antorbital fenestra (as in theropods) to reach the primary choana.
Thus, the specified osteological correlates are present and
in fact quite similar to more basal crocodylomorphs.
Araripesuchus gomesii (Fig. 28) is very closely related to the
clade of higher metasuchians called Neosuchia (Benton and