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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
WITMERANTORBITAL CAVITY OF ARCHOSAURS lral I - . ... rad / dent fen D I rec fl;\'' - - \..rA..Hn.I pila ~IUlIldA par med sin max FIGURE 29. The accessory cavities in the maxilla, lacrimal, and/or nasal of two theropods. A, Allosaurus fragilis, antorbital region in left lateral view. B, left maxilla of same in medial view. C, Marshosaurus bicentesimus, left maxilla in lateral view. D, same in medial view. (A,B modified after Madsen [1976b] and specimens; C,D reconstructed from UUVP 1846 and UUVP 4695.) Clark, 1988). This is significant because some metasuchians more basal than A. gomesii have closed the external antorbital fenestra. Among these are Baurusuchus pachecoi (Price, 1945), Sebecus icaeorhinus, and probably Libycosuchus brevirostris (Stromer, 1914; Buffetaut, 1982). Benton and Clark (1988) regarded this reduction as a synapomorphy of these taxa and neosuchians, with the condition in A. gomesii being a reversal. However, Gasparini et al. (1991) placed A. gomesii more basally and considered closure of the fenestra a synapomorphy of B. pachecoi and S. icaeorhinus. Clearly this segment of crocodylomorph phylogeny needs additional work, as noted by Benton and Clark (1988). As its name implies, Neosuchia includes crocodylomorphs of genuinely modem aspect. This modem appearance results from the characteristic pattern of skull flattening in which structures, such as the lacrimal, that were laterally placed in more basal forms are now located dorsally on the roof of the snout. Thus, it is possible that the morphogenetic mechanism producing this pattern in extant crocodilians-i.e., rotation of the nasal cavity (Witmer, 1995b)-arose in the common ancestor of Neosuchia. Most non-crocodilian neosuchians were not examined in great detail for this study, but comments will be made on a few of them. Retention of an (external) antorbital fenestra was regarded by Norell and Clark (1990) as a plesiomorphy of Atoposauridae, the basal clade of neosuchians, although Clark (1986) and Buscalioni and Sanz (1988) noted its absence in some atoposaurids. The atoposaurid Theriosuchus pusillus almost certainly has a small opening between the maxilla and lacrimal and even appears to have a thalattosuchian-like groove on the maxilla leading rostrally from it (BMNH 48330, 48260). The maxillary groove in T. pusillus may signal, as in Pelagosaurus typus, the
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