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WITMER-ANTORBITAL CAVITY OF ARCHOSAURS 27<br />
nal antorbital fenestra. In Tyrannosaurus rex (CM 9401; see<br />
also Molnar, 1991), the canal apparently traverses only the short<br />
distance through the vertical jugal ramus between orbit and antorbital<br />
cavity. Other variants could be cited, but, despite this<br />
variation, it is clear that non-avian theropods exhibit the same<br />
general relations as observed in extant archosaurs. As in present-day<br />
birds, the duct ran through the most dorsal portion <strong>of</strong><br />
the internal antorbital fenestra.<br />
Dinosauria: Sauropodomorpha-In virtually all sauropodomorphs,<br />
the internal antorbital fenestra opens into the nasal<br />
cavity opposite the choana. Laterally, the antorbital cavity is<br />
bounded principally by the lacrimal and maxilla, with varying<br />
contributions from the nasal (some prosauropods) or from the<br />
jugal (some sauropods). The relationship <strong>of</strong> fenestra with choana<br />
can be observed in Plateosaurus engelhardti (AMNH 6810;<br />
Fig. 12), probably Thecodontosaurus antiquus (Kermack,<br />
1984), Massospondylus spp. (Attridge et al., 1985; Gow et al.,<br />
1990), Lufengosaurus huenei (Young, 1941), Brachiosaurus<br />
brancai (Janensch, 1935-36), Euhelopus zdanskyi (Mateer and<br />
McIntosh, 1985), Carnarasaurus lentus (CM 11338; Fig. 13),<br />
Shunosaurus lii (Zhang, 1988), Orneisaurus tianfuensis (He et<br />
a]., 1988), and Diplodocus longus (CM 11161, 3452).<br />
Few if any sauropodomorphs have the large external antorbital<br />
fossae that are so commonly observed in more basal archosaurs.<br />
Rather, there seems to be a trend to enclose the antorbital<br />
cavity in prosauropods, and distinguishing between internal<br />
and external antorbital fenestrae is <strong>of</strong>ten necessary. In<br />
most prosauropods, the supralveolar lamina <strong>of</strong> the maxilla is<br />
present and encloses a portion <strong>of</strong> the antorbital cavity medial<br />
to it. This lamina is unknown for Thecodontosaurus antiquus<br />
(Kermack, 1984) and is virtually absent in Anchisaurus polyzelus<br />
(YPM 1883; see also Galton, 1976: fig. 13); it is welldeveloped<br />
in Plateosaurus engelhardti (AMNH 6810; Fig.<br />
12A), Massospondylus carinatus (Gow et al., 1990), and Lufengosaurus<br />
huenei (Young, 1941), but very low and <strong>of</strong> limited<br />
caudal extent in Sellosaurus gracilis (Galton, 1985b). Similarly,<br />
the ascending ramus <strong>of</strong> the maxilla tends to send lateral and<br />
medial laminae caudally. The short lateral lamina is continuous<br />
with the supralveolar lamina, and the two together form the<br />
sharp margin <strong>of</strong> the external antorbital fenestra. Sauropodomorphs<br />
synapomorphically reduce the medial lamina <strong>of</strong> the ascending<br />
ramus (Sereno, 1989). In prosauropods, the medial<br />
lamina is variably developed, ranging from being very short in<br />
Anchisaurus polyzelus (YPM 1883), short to moderately developed<br />
in Massospondylus carinatus (Gow et al., 1990), Coloradisaurus<br />
brevis (Bonaparte, 1978), Sellosaurus gracilis (Galton,<br />
1985b), and Lufengosaurus huenei (Young, 1941), to very<br />
extensive in Plateosaurus engelhardti (AMNH 6810; Fig. 12).<br />
The caudal extent <strong>of</strong> the medial lamina largely determines the<br />
size <strong>of</strong> the internal antorbital fenestra. All these laminae enclose<br />
the maxillary antorbital fossa.<br />
The lacrimal <strong>of</strong> prosauropods has a ventrolateral antorbital<br />
fossa resembling many theropods (e.g., Thecodontosaurus antiquus,<br />
Kermack, 1984; Massospondylus carinatus, Gow, et al.,<br />
1990; Sellosaurus gracilis, Galton, 1985b; Lufengosaurus huenei,<br />
Young, 1941 ; Plateosaurus engelhardti, AMNH 6810,<br />
Fig. 12A). The bone usually twists somewhat along its length<br />
such that the fossa is carried dorsomedially where, in at least<br />
Plateosaurus engelhardti (AMNH 6810; Fig. 12) and apparently<br />
also Sellosaurus gracilis (Galton, 1985b), there is an ex-<br />
pansive cavity-part <strong>of</strong> the antorbital cavity. The maxilla and<br />
lacrimal contact each other above the antorbital cavity in probably<br />
all prosauropods (Galton, 1990). Furthermore, in Plateosaurus<br />
engelhardti (AMNH 6810), possibly in Sellosaurus<br />
gracilis (Galton, 1985b), and in Massospondylus carinatus<br />
(Gow et al., 1990), the nasal bone overhangs the antorbital cavity<br />
and may house a small accessory cavity (Fig. 12C; see section<br />
on accessory cavities below). Thus, the antorbital cavity in<br />
prosauropods is clearly associated with well-marked excavations<br />
<strong>of</strong> the maxilla, lacrimal, and, at least sometimes, nasal,<br />
which, in some derived forms, enclose substantial portions <strong>of</strong><br />
the cavity.<br />
The antorbital cavities <strong>of</strong> sauropods generally are relatively<br />
small (Fig. 13). In most forms, retraction <strong>of</strong> the naris has displaced<br />
the cavity ventrally, and the dorsal portion <strong>of</strong> the antorbital<br />
fenestra <strong>of</strong>ten has a pinched appearance (McIntosh, 1990).<br />
Most sauropods have well-developed supralveolar laminae giving<br />
a sharp ventral edge to their antorbital fenestrae (e.g., Brachiosaurus<br />
brancai, Janensch, 1935-36; Patagosaurus fariasi,<br />
Bonaparte, 1986; Euhelopus zdanskyi, Mateer and McIntosh,<br />
1985; Camarasaurus lentus, CM 11338, 11969, 41694, 21702,<br />
113; adult Shunosaurus lii, Zhang, 1988; Orneisaurus tianfuensis,<br />
Dong et al., 1983; Apatosaurus louisae, CM 11 162; Diplodocus<br />
longus, CM 1 1 161, 1 1255, 3452; Nemegtosaurus<br />
rnongoliensis, Nowinski, 1971). The supralveolar lamina thus<br />
partly encloses the antorbital cavity so that the aforementioned<br />
taxa have a prominent antorbital fossa on the medial surface <strong>of</strong><br />
the maxilla (unknown for Apatosaurus louisae and not readily<br />
comparable in Diplodocus longus). The ascending ramus <strong>of</strong> the<br />
maxilla is a thin process in most sauropods (Fig. 13A) and does<br />
not develop the medial and lateral lamina seen in higher prosauropods.<br />
Retraction <strong>of</strong> the naris and expansion <strong>of</strong> the nasal<br />
vestibule has carried the ascending ramus caudodorsally, presumably<br />
tracking the maxillary (subnarial) process <strong>of</strong> the nasal<br />
with which it articulates (except in Nemegtosaurus mongoliensis;<br />
Nowinski, 1971). Probably as a result, the rostral ramus <strong>of</strong><br />
the lacrimal is almost absent, and the lacrimal is usually a simple<br />
pillar, with little or no development <strong>of</strong> an antorbital fossa.<br />
Thus, the antorbital cavities <strong>of</strong> most sauropods are relatively<br />
simple.<br />
However, Diplodocus longus and presumably also its more<br />
poorly known relatives have a relatively complex snout (Fig.<br />
17). The most obvious features are the relatively long antorbital<br />
fenestra and the preantorbital fenestra rostral to it, the two being<br />
separated by an interfenestral bridge (CM 11 161, 11255, 3452).<br />
The preantorbital fenestra is reminiscent <strong>of</strong> the maxillary fenestra<br />
<strong>of</strong> theropods, but unlike the latter it is not recessed within<br />
a single large fossa. Rather, the interfenestral bridge is at the<br />
same level as the surrounding surface. The preantorbital fenestra<br />
itself, however, is clearly set in a distinct fossa rostroventral<br />
to the fenestra; the fossa is somewhat better developed in a<br />
juvenile skull <strong>of</strong> D. longus (CM 11255; Fig. 17A, B). There is<br />
also a shallower, triangular fossa extending caudally ventral to<br />
the interfenestral bridge. The preantorbital fenestra and fossa<br />
open medially into a space continuous with the antorbital cavity.<br />
Thus, the antorbital cavity extends rostrally medial to the<br />
interfenestral bridge and dorsally over a strong medial shelf<br />
formed by the palatal process <strong>of</strong> the maxilla and palatine and<br />
buttressed by the ectopterygoid and pterygoid wing (Holland,<br />
1924; McIntosh and Berman, 1975) to open laterally again via<br />
the preantorbital fenestra (Fig. 17C).<br />
A likely explanation for the unique facial structure <strong>of</strong> Diplodocus<br />
longus is not the apomorphic appearance <strong>of</strong> the preantorbital<br />
fenestra, but rather the interfenestral bridge. In other<br />
words, the long lateral aperture <strong>of</strong> the antorbital cavity becomes<br />
subdivided by the bridge. The bridge probably served to buttress<br />
the long, low snout; it is significant in this regard that the<br />
medial shelf complex is situated at the ventral end <strong>of</strong> the bridge.<br />
Whether or not other diplodocids exhibit this system is not<br />
clear. A preantorbital fenestra may be present in Apatosaurus<br />
louisae (CM 1 1162; Berman and McIntosh, 1978). Dicraeosaurus<br />
hansemanni (Janensch, 1935-36) shows no sign <strong>of</strong> either<br />
fenestra in the fragmentary maxillary material, although<br />
the preserved lacrimal suggests that an antorbital fenestra was<br />
present. Nernegtosaurus rnongoliensis (Nowinski, 197 1) also