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3 6 SOCIETY OF VERTEBRATE PALEONTOLOGY, MEMOIR 3 FIGURE 24. below. Dibothrosuchus elaphros. Stereophotographs of the snout of IVPP V7907 in right dorsolateral view, with interpretive drawing
WITMER-ANTORBITAL CAVITY OF ARCHOSAURS 37 I \ sulc max fen antorb int Lg wall cav paraias ch FIGURE 26. Pelagosaurus typus. A, drawing of antorbital region of BMNH 32599 in left lateral view. B, schematic drawing of a medial view of the right antorbital region of the same specimen (observed through the orbit directly and with dental mirror). pter FIGURE 25. Protosuchian crocodylomorphs. A, Protosuchus richardsoni, snout of MCZ 6727 (with some details modified from Crompton and Smith [I9801 and Sues et al. [1994]) in left lateral view. B,C, Gobiosuchus kielanae, snout in (B) left lateral and (C) ventral views. (B,C modified after Osmolska, 1972.) 1962). Metriorhynchids have a small internal antorbital fenestra, bounded by the maxilla and lacrimal and often the nasal, but usually have a much more extensive antorbital fossa (Fig. 27). Whereas the external antorbital fenestra is often continued rostrally as a narrow groove in P. typus and some teleosaurids (Fig. 26A), there is always a broad antorbital fossa in metriorhynchids (bounded by the maxilla and lacrimal and usually also the nasal and jugal), excavated into the side of the snout rostral to the fenestra and continued rostrally as a tapering groove on the maxilla (Fig. 27; Wenz, 1968; Gasparini and Chong Diaz, 1977; Buffetaut, 1982). There is considerable intras~ecific variation in the form of the fossa and groove. For example, in Metriorhynchus superciliosus, the boundary between fossa and groove may be relatively subtle (BMNH R3014, R3900, R4762), or the fossa may be very deeply excavated and extensive with the rostral groove being distinct from (although continuous with) the rostral edge of the fossa (BMNH R3899). Also in this species and in Teleidosaurus gaudryi (BMNH R3353), the antorbital fossa clearly extends caudally onto the lacrimal (BMNH R3900, R4762; see Fig. 27). The relationship between the primary choana and internal antorbital fenestra is known in P. typus (BMNH R32599) and M. superciliosus (BMNH R2048; see also Wenz, 1968) where the fenestra clearly opens medially into the nasal cavity and is directly opposite the primary choana. An unequivocal nasolacrimal canal could not be located in even the well-preserved material of P. typus, Steneosaurus spp., and M. superciliosus; the only possible candidate is a matrix-filled pit within the orbital margin of the lacrimal in Teleidosaurus gaudryi (BMNH
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