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