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WITMERANTORBITAL ( 7AVITY OF ARCHOSA URS 35<br />
fen antorb int<br />
suborb<br />
lac<br />
pier<br />
FIGURE 23. Postosuchus kirkpatricki, facial skeleton. A, left lateral<br />
view. B, ventral view. (Redrawn from Chatterjee, 1985.)<br />
by an extensive, well-marked antorbital fossa (Fig. 23). In G.<br />
stipanicicorum, however, there is no fossa on the lateral surface<br />
<strong>of</strong> the jugal ramus <strong>of</strong> the lacrimal or ventrally on the body <strong>of</strong><br />
the maxilla. Desmatosuchus haplocerus (TTUP 9023; see also<br />
Case, 1922) is also an exception in that the fossa does not<br />
extend ventrally onto the maxilla.<br />
The course <strong>of</strong> the nasolacrimal canal is known in the stagonolepidids<br />
D. haplocerus (TTUP 9023) and Stagonolepis robertsoni<br />
(Fig. 9A,B; Walker, 1961) where it clearly opens orbitally<br />
between the lacrimal and prefrontal. The rostra1 course <strong>of</strong><br />
the duct is a little less clear. It probably did not extend to the<br />
naris as suggested by Walker (1961) but rather opened into the<br />
choanal recess medial to the ascending ramus <strong>of</strong> the maxilla<br />
(e.g., S. robertsoni, BMNH R4787), passing dorsomedially<br />
around the antorbital cavity. The nasolacrimal canal is present<br />
in Postosuchus kirkpatricki (TTUP 9000; Chatterjee, 1985), extending<br />
from orbit to antorbital cavity, but it is not clear if it<br />
runs within the lacrimal or in the lacrimoprefrontal suture.<br />
Crurotarsi: Crocodylomorpha-The general conformation<br />
<strong>of</strong> the facial bones in basal crocodylomorphs does not differ<br />
substantially from that observed in most other archosaurs, although<br />
the trend for reduction and enclosure <strong>of</strong> the antorbital<br />
cavity is already evident (see section on facial trends below).<br />
The basal sphenosuchians Terrestrisuchus gracilis (Crush,<br />
1984), Saltoposuchus connectens (Huene, 1921 ; Sereno and<br />
Wild, 1992), and Pseudhesperosuchus jachaleri (Bonaparte,<br />
1972) retain a relatively large antorbital cavity that excavates a<br />
deep antorbital fossa on the extensive medial laminae <strong>of</strong> the<br />
maxilla and lacrimal. Thus, the internal antorbital fenestra, bor-<br />
dered by the lacrimal and maxilla, is relatively long and low<br />
and is much smaller than the external fenestra. A well-developed<br />
supralveolar lamina provides a sharp ventral border to the<br />
fenestra, and the palatine partially floors the antorbital cavity.<br />
The internal antorbital fenestra opens directly opposite the choana<br />
within the nasal cavity, as in all sphenosuchians (Figs. 1,<br />
24).<br />
The course <strong>of</strong> the nasolacrimal canal is known best in Sphenosuchus<br />
acutus where it passes dorsomedially over the antorbital<br />
cavity completely within the lacrimal, just internal to the<br />
antorbital fossa, to open medially above the dorsal portion <strong>of</strong><br />
the internal antorbital fenestra; the duct presumably continued<br />
to the choana along a low ridge on the medial surface <strong>of</strong> the<br />
ascending ramus <strong>of</strong> the maxilla (Walker, 1990). The position <strong>of</strong><br />
the orbital aperture <strong>of</strong> the canal in Terrestrisuchus gracilis<br />
(Crush, 1984), Saltoposuchus connectens (Sereno and Wild,<br />
1992), and Dibothrosuchus elaphros (IVPP V7907; Fig. 24)<br />
implies a similar course in these animals.<br />
Basal crocodyliforms (protosuchians) continue the trend <strong>of</strong><br />
reduction <strong>of</strong> the antorbital cavity and external antorbital fenestra.<br />
In the forms for which data are available, the internal antorbital<br />
fenestra is a relatively small opening bounded by the<br />
lacrimal and maxilla (Fig. 25A). It clearly opens medially into<br />
the nasal cavity, and it can be seen to be opposite the choana<br />
in Protosuchus richardsoni (MCZ 6727, UCMP 130860,<br />
AMNH 3024; Crompton and Smith, 1980; Clark, 1986), an unnamed<br />
protosuchid (UCMP 97638; Clark, 1986), Orthosuchus<br />
stormbergi (Nash, 1975), Gobiosuchus kielanae (Osmblska,<br />
1972; Fig. 25B,C), and Hoplosuchus kayi (CM 11361). The<br />
antorbital cavity is floored medially by the palatine in at least<br />
Gobiosuchus kielanae (Osm6lska, 1972; Fig. 25B,C). The antorbital<br />
fossa is not extensive in any <strong>of</strong> these taxa, but tends to<br />
excavate the lacrimal and maxilla caudally, dorsally, and rostrally.<br />
It is not clear if the apparently prominent maxillary fossa<br />
<strong>of</strong> Hemiprotosuchus leali (Bonaparte, 1972) or the "accessory<br />
antorbital depression" <strong>of</strong> Platyognathus hsui (Wu and Sues,<br />
1996) are associated with the antorbital cavity. The course <strong>of</strong><br />
the nasolacrimal canal has been described only for Protosuchus<br />
richardsoni, where it extends within the lacrimal from the orbit,<br />
dorsomedially over the antorbital cavity, to open into the caudodorsal<br />
margin <strong>of</strong> the internal antorbital fenestra (Clark,<br />
1986).<br />
Further reduction <strong>of</strong> the antorbital cavity and closure <strong>of</strong> the<br />
external antorbital fenestra occurs more than once in Mesoeucrocodylia,<br />
how many times depending on the cladogram adopted<br />
(see below). Before surveying this taxon, one <strong>of</strong> the osteological<br />
correlates, the position <strong>of</strong> the choana, requires clarification.<br />
The most obvious synapomorphy <strong>of</strong> Mesoeucrocodylia<br />
involves the formation <strong>of</strong> a bony nasopharyngeal duct such that<br />
the opening <strong>of</strong> the airway is diverted caudally (Huxley, 1875;<br />
Langston, 1973; Benton and Clark, 1988; Clark, 1994; Busbey,<br />
1995). This opening usually is referred to as the "choana."<br />
However, as mentioned above for extant crocodilians, the rostral<br />
end <strong>of</strong> the nasopharyngeal duct is the primary choana, homologous<br />
to the choana <strong>of</strong> other archosaurs (Witmer, 1995b),<br />
and the caudal end <strong>of</strong> the duct is a mesoeucrocodilian neomorph,<br />
the secondary choana. Thus, in the following discussion,<br />
explicit reference will be to only the primary choana.<br />
The basal mesoeucrocodilian clade (Buffetaut, 1982; Benton<br />
and Clark, 1988; Wu, Li, and Li, 1994), Thalattosuchia, represents<br />
the earliest instance <strong>of</strong> extensive reduction <strong>of</strong> the antorbital<br />
cavity. The basal thalattosuchian Pelagosaurus typus<br />
(Fig. 26; BMNH 32599) resembles teleosaurids such as Steneosaurus<br />
spp. (FMNH UC 402, CM 360, MCZ 1063, many<br />
BMNH specimens) in having a very small, slitlike external antorbital<br />
fenestra between the maxilla and lacrimal with very<br />
little to no development <strong>of</strong> an antorbital fossa (Fig. 26A); in<br />
some specimens the fenestra may even be absent (see Westphal,