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WITMER-ANTORBITAL<br />
extensive medial lamina <strong>of</strong> the maxillary ascending ramus and<br />
a similar medial lamina from the lacrimal bone such that there<br />
is an extensive antorbital fossa (Figs. 7, 8). These thin medial<br />
laminae are well preserved in Lesothosaurus diagnosticus<br />
(BMNH RUB23, R11956, RUB17, R8501), Heterodontosaurus<br />
tucki (BMNH R8179 [cast <strong>of</strong> SAM 337]), and Hypsilophodon<br />
foxii (BMNH R197, R2477) and constrict the size <strong>of</strong> the internal<br />
antorbital fenestra to a foramen. The maxillary medial larnina<br />
<strong>of</strong> Hypsilophodon foxii (BMNH R2477) and also Heterodontosaurus<br />
tucki (but not that <strong>of</strong> the basal heterodontosaurid<br />
Lanasaurus scalpridens; Gow, 1975) has an additional opening<br />
between the antorbital and nasal cavities.<br />
The antorbital cavity is enclosed laterally to a variable degree.<br />
In all the aforementioned taxa, there is a well-developed<br />
supralveolar lamina projecting dorsally, giving the ventral margin<br />
<strong>of</strong> the external antorbital fenestra a sharp edge. In Lesothosaurus<br />
diagnosticus there is no significant development <strong>of</strong> a<br />
lateral lamina <strong>of</strong> the maxillary ascending ramus; the rim around<br />
the extensive antorbital fossa is sometimes distinct (BMNH<br />
R11956) and sometimes not (BMNH RUB17). In Heterodontosaurus<br />
tucki and probably Abrictosaurus consors, but not in<br />
Lanasaurus scalpridens, the rostral rim <strong>of</strong> the antorbital fossa<br />
more strongly overhangs the cavity. In Hypsilophodon foxii, the<br />
lateral lamina <strong>of</strong> the ascending ramus is extensive and greatly<br />
restricts the size <strong>of</strong> the external antorbital fenestra (Fig. 8B). In<br />
all taxa, the lacrimal also develops a lateral lamina that overhangs<br />
and further encloses the cavity (Figs. 7, 8).<br />
The course <strong>of</strong> the nasolacrimal duct is known in Lesothosaurus<br />
diagnosticus (BMNH R8501) and Hypsilophodon foxii<br />
(BMNH R2477). In the latter (Fig. 8A), the canal passes<br />
through the lacrimal from the orbit, through the rostral ramus,<br />
emerging at the rostral end medial to the medial lamina <strong>of</strong> the<br />
maxilla as observed in Plateosaurus engelhardti (Fig. 12D). In<br />
Lesothosaurus diagnosticus, only the orbital opening <strong>of</strong> the canal<br />
is visible but the canal clearly does not open rostrolaterally<br />
and therefore probably had a course similar to that in Hypsilophodon<br />
.foxii. Thus, in both cases, the nasolacrimal duct<br />
passed dorsomedially around the antorbital cavity.<br />
Finally, Lesothosaurus diagnosticus has an additional fossa<br />
on the palatine medial to the choana (mentioned above in the<br />
discussion <strong>of</strong> the muscular hypothesis; Fig. 7C, D). This fossa<br />
perhaps supported the nasal capsule, but it is more likely that<br />
it is an accessory recess associated with the antorbital cavity<br />
(see section on accessory cavities below).<br />
In higher ornithopods, many <strong>of</strong> the aforementioned trends are<br />
carried further. In Tenontosaurus tilletti (MOR 682), Dryosaurus<br />
altus (CM 3392; see also Galton, 1983), and Carnptosaurus<br />
dispar (UUVP 5946) the antorbital cavity is further reduced in<br />
size, and lateral laminae from the maxilla and lacrimal (and<br />
jugal in D. altus) constrict the external antorbital fenestra to a<br />
relatively small aperture. The same relationships <strong>of</strong> antorbital<br />
cavity to choana that were observed in more basal ornithopods<br />
and Lesothosaurus diagnosticus are present in the iguanodontians<br />
Iguanodon ather-eldensis (BMNH R5764, 1 152 1 ; Norman,<br />
1986; Fig. ll), probably I. lakotaensis (SDSM 8656<br />
[cast]; Weishampel and Bjork, 1989), and Ouranosaurus nigeriensis<br />
(MNHN GDF 300 [cast]; Taquet, 1976), but here the<br />
antorbital cavity is very small, does- not deeply excavate the<br />
maxilla or lacrimal, and is enclosed laterally by only the lacrimal<br />
such that a faint fossa extends a short distance rostrally<br />
on the maxilla. Where known, the nasolacrimal canal passes<br />
dorsomedially around the antorbital cavity (Fig. 11B). Hadrosaurids<br />
present an interesting case in that they have a completely<br />
closed external antorbital fenestra, which is walled in not<br />
only by the massive maxilla, but also by the lacrimal, premaxilla,<br />
and a large rostral process <strong>of</strong> the jugal (Weishampel and<br />
Homer, 1990). The antorbital cavity, however, is retained medially<br />
as a small cavity bounded mostly by maxilla, lacrimal,<br />
CAVITY OF ARCHOSAURS<br />
lac<br />
fen antorb -<br />
rnax<br />
FIGURE 18. Ernausaurus emsti, facial skeleton in left lateral view.<br />
(Modified after Haubold, 1990.)<br />
and palatine; it is open caudally and floored ventrally with foramina<br />
for passage <strong>of</strong> neurovasculature. The cavity is directly<br />
opposite the choana. It should be noted that Heaton (1972) positioned<br />
the choana much too far caudally, behind the vomer;<br />
instead, the choana <strong>of</strong> hadrosaurids is lateral to the vomer,<br />
bounded caudally by palatine and laterally by maxilla as in<br />
other ornithopods.<br />
Dinosauria: Thyreophora-Thyreophoran ornithischians<br />
also tend to reduce the antorbital cavity and close the external<br />
antorbital fenestra. Unfortunately, little relevant data are available<br />
in the published material <strong>of</strong> the basal thyreophoran Scutellosaurus<br />
lawleri (Colbert, 1981). The slightly more derived<br />
thyreophoran, Ernausaurus ernsti (Haubold, 1990), provides<br />
critical data. In this taxon, a well-developed antorbital cavity is<br />
formed within the maxilla and lacrimal (Fig. 18). As in Lesothosaurus<br />
diagnosticus and other forms, the maxilla and lacrimal<br />
have broad medial laminae constricting the internal antorbital<br />
fenestra to a relatively small opening. Although the palatal<br />
elements are not known, the medial surface <strong>of</strong> the maxilla<br />
shows the palatine articular surface, thus fixing the position <strong>of</strong><br />
the choana as directly opposite the internal antorbital fenestra.<br />
There is a low supralveolar lamina (Haubold, 1990), and the<br />
maxilla and lacrimal both have lateral laminae constricting the<br />
size <strong>of</strong> the external antorbital fenestra. Significantly, Haubold<br />
(1990) described a deep cavity entering the maxilla from the<br />
antorbital cavity. The nasolacrimal canal passes through the lacrimal<br />
bone, dorsomedial to the antorbital cavity.<br />
Scelidosaurus harrisonii, the next higher thyreophoran, is<br />
generally similar but shows greater closure <strong>of</strong> the cavity. In S.<br />
harrisonii (BMNH R111 l), the internal antorbital fenestra is<br />
not only directly opposite the choana, but the postchoanal strut<br />
<strong>of</strong> the palatine contributes to the caudal margin <strong>of</strong> the fenestra.<br />
The massive maxilla is so transversely thick that there is actually<br />
a short canal passing laterally from the internal fenestra<br />
to the main cavity. The antorbital cavity is enclosed by the<br />
maxilla, lacrimal, and probably jugal. There is a distinct antorbital<br />
fossa well excavated into the external surface <strong>of</strong> the max-