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SOCIETY OF VERTEBRATE PALEONTOLOGY, MEMOIR 3 Sphenosuchus Araripesuchus Alligator P.ce~nJhesnerosuchus I Theriosuchus I I 1 Mesoeucrocodylia 1 Crocodylomorpha FIGURE 37. Facial trends in Crocodylomorpha. In the course of crocodylomorph evolution, the external antorbital fenestra becomes reduced and eventually closed, and the antorbital cavity likewise becomes reduced and eventually completely internalized. These trends are associated with restriction of the main paranasal air sinus in connection with the development of maxillary palatal processes and dorsoventral flattening of the snout. Skull drawings modified from Owen (1878), Rusconi (1932), Kalin (1955), Bonaparte (1972), Gasparini (1971), Buffetaut (1982), Walker (1990), and Hecht (199 1). to become smaller, eventually closing such that the antorbital cavity is completely internalized within the snout. There are a couple of concurrent trends that also need to be considered here: first, the formation of a so-called "secondary palate," and second, dorsoventral flattening of the skull. The first of these itself subsumes two probably separate phenomena: (1) formation of palatal processes of the maxilla and (2) formation of a bony nasopharyngeal duct. Of these last two, the formation of more extensive palatal processes of the maxilla is, in fact, correlated with the initial reduction of the antorbital cavity. Expansion of the maxillary palatal processes (e.g., in Sphenosuchus acutus or Protosuchus richardsoni relative to
IVITMER-ANTORBITAL Pseudhesperosuchus jachaleri) diverted the primary choana caudally. Because of the fundamental morphogenetic relationship between the primary choana and antorbital sinus (discussed above; see also Witmer, 1995b), the entire system shifted caudally where, encroaching on the orbit and its contents, there simply was less space available for the antorbital cavity; in other words, it was constrained by "packing" phenomena. It turns out, however, that formation of a nasopharyngeal duct in mesoeucrocodilians seems to have had virtually no effect on the subsequent reduction or enclosure of the antorbital cavity, because forms such as Notosuchus terrestris, Uruguaysuchus aznarezi, and Araripesuchus gomesii retain an antorbital cavity similar to that of protosuchians. This situation probably results because the development of a nasopharyngeal duct involves the presence and position of the secondary choana, not the primary choana (Witmer, 1995b); the primary choana (i.e., the rostra1 end of the duct) had not moved relative to the antorbital cavity. In fact, all this is further evidence affirming the causal relationship of primary choana and antorbital sinus. So, if the initial reduction of the antorbital cavity perhaps has its causal basis in the evolution of palatal processes of the maxilla, what factors are involved in the ultimate reduction of the cavity? As discussed earlier, extreme reduction and closure of the external antorbital fenestra occurs multiple times in Crocodylomorpha: once or twice in Thalattosuchia, at least once or twice among basal metasuchians, and at least once or twice in Neosuchia. Concomitant internalization of the antorbital cavity occurred clearly at least twice: at least once in Thalattosuchia and at least once in Neosuchia. In most of these cases, closure of the external fenestra can be shown to be associated with apomorphic flattening of the snout. Skull flattening, the other imuortant concurrent trend mentioned above, has been well documented by Langston (1973) and especially Busbey (1995). The biomechanical consequences of dorsoventral flattening are considerable. Flattening the snout (Fig. 38B) moves it away from the design optimum of a cylinder (Fig. 38A), potentially making it less competent to resist sagittal bending and torsional loads (Witmer, 1992b). Busbey's (1995) elegant functional analysis of the trend from oreinirostral (tall-snouted) sphenosuchians and protosuchians to platyrostral (flat-snouted) neosuchians confirmed these mechanical sequelae and suggested mechanisms to resist these stresses. In particular, Busbey (1995) noted that platyrostral taxa show increased cross-sectional area of bone through (1) thickening of the bones and (2) the development of a secondary palate (interestingly, even an incomplete secondary palate has biomechanical benefits). Furthermore, Busbey (1995) was correct in noting that platyrostral skulls loaded in sagittal bending exhibit stress concentrations at the caudal end of the snout, just in front of the orbits. These stress concentrations are in precisely the position of the external antorbital fenestrae. Therefore, an external fenestra severely decreases the ability of the snout to resist these torsional and especially sagittal loads, because such a gap or discontinuity would produce a so-called "open section" (Fig. 38B). In fact, it would result in the coincidence of an open section and a stress concentration-a potentially catastrophic design. Therefore, I would suggest (see also Witmer, 1992b) that the ultimate closure of the external fenestra is causally linked to platyrostry as another mechanism to increase cross-sectional area of bone. Thus, in light of the epithelial hypothesis for the function of pneumaticity, the evolutionary trends in the snouts of crocodylomorphs support the claim of a "struggle" between the conflicting tendencies of pneumatization and maintenance of adequate strength. The caudal shift of the primary choana resulting from the development of maxillary palatal processes pushed the whole pneumatic system caudally and constricted it as it competed for space with the orbital contents. Nevertheless, the cavity remained tolerably large in the oreinirostral metasuchians CAVITY OF ARCHOSAURS A primitive condition: cylindrical snout fen antorb ext "dangerous" derived condition: dorsoventrally flattened snout retaining external fenestra agitta bending open cross section produced by fenestra fen antorb ext FIGURE 38. Biomechanical implications of platyrostry (flattening of the snout). A, the primitive condition (manifested by sphenosuchians, protosuchians, many basal metasuchians) is to have a much taller, more cylindrical snout. B, the dorsoventral flattening observed in some clades (most notably neosuchians) makes the snout particularly susceptible to failure under sagittal bending andlor torsion. Furthermore, the stress concentrations resulting from platyrostry are in precisely the locations of the external antorbital fenestra, which would produce an open cross section further weakening the snout. Ultimate closure of the external antorbital fenestra and internalization of the antorbital cavity was probably largely a biomechanical consequence of platyrostry. such as Araripesuchus gomesii. However, the flattening of the snout produced a biomechanical weak spot at exactly the location of the external antorbital fenestra. Thus closure of the external fenestra and internalization of the paranasal air sac was a biomechanical solution to the problem (and nasal rotation was probably the morphogenetic mechanism accomplishing this closure; Witmer, 1995b). It would seem that in crocodylomorphs, the paranasal air sinus "loses" in the metaphorical struggle, and this is probably a fair conclusion. But, interestingly, in the broad- but flat-snouted alligatorines, pneumaticity has rebounded, and the snout is a multi-chambered maze with stout bony struts at biomechanically predictable locations. Ornithopoda-Turning to ornithischian dinosaurs, the focus will be Omithopoda, but most major clades of ornithischians
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