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30 SOCIETY OF VERTEBRATE PALEONTOLOGY, MEMOIR 3
illa; presumably this entire portion of the maxilla is the homolog
of the medial lamina of other ornithischians. In the rostral
apex of the antorbital fossa is a moderately large foramen
directed rostrally into the bone. It is unknown if the foramen
expands into an accessory cavity comparable to the cavity described
for Emausaurus ernsti. As in the latter taxon, the nasolacrimal
duct passes dorsomedially around the antorbital cavity
through the lacrimal bone.
Among stegosaurs, the basal taxon Huayangosaurus taibaii
exhibits a small but distinct external antorbital fenestra within
the maxilla, lacrimal, and jugal (see Sereno and Dong, 1992).
The internal antorbital fenestra is caudally situated within the
antorbital cavity and is probably opposite the choana. A welldeveloped
medial lamina of the maxilla walls the cavity internally,
forming an antorbital fossa that undercuts the dorsal margin
of the external antorbital fenestra. The cavity is partially
walled-in laterally by a prominent supralveolar lamina and a
lateral lamina of the lacrimal. The external antorbital fenestra
is absent in Chungkingosaurus jiangbeiensis (Dong et al., 1983)
and Tuojiangosaurus multispinus (Dong et al., 1983). It is present,
however, in Stegosaurus stenops, in which the antorbital
cavity is similar to Huayangosaurus taibaii but smaller and
shallower (Sereno and Dong, 1992).
The Ankylosauria are a highly apomorphic group: they lack
an external antorbital fenestra, most of the skull sutures have
been obliterated by overlying dermal ossifications, and their
snouts enclose a complicated pattern of cavities (Coombs, 1971,
1978; Maryanska, 1977; Coombs and Maryanska, 1990). As a
result, the specified osteological correlates are difficult to assess.
There clearly is a cavity within the maxillae of probably
all ankylosaurids that may be homologous with the antorbital
cavity (e.g., Euoplocephalus tutus, AMNH 5843, Fig. 19A; Ankylosaurus
magniventris, AMNH 5894; see also Maryanska,
1977; Coombs, 1971, 1978; Tumanova, 1987; Coombs and
Maryanska, 1990). This cavity-the "maxillary sinus" of the
aforementioned authors-communicates rostrally with the narial
region via a foramen in the premaxilla ventral or lateral to
the true naris (e.g., Pinacosaurus grangeri, AMNH 6523; Euoplocephalus
tutus, AMNH 5843; Ankylosaurus magniventris,
AMNH 5214, 5895; see Maryanska, 1977). Maryanska (1977)
suggested that the maxillary sinus lodged the glandula nasalis,
which is not unreasonable since the cavity opens into the caudal
part of the naris; furthermore, such a communication between
the antorbital cavity and naris would be unique. However, the
cavity seems inordinately large for a gland, and it communicates
with other paranasal cavities (e.g., in Ankylosaurus magniventris,
AMNH 5895). Moreover, Maryanska (1977) described
a vertical septum within this cavity in Pinacosaurus
grangeri and noted that both parts of the cavity open into the
nasal cavity proper. Perhaps strictly the caudal portion of the
cavity is the homolog of the antorbital cavity. Unfortunately,
the communications of the various sinuses with each other and
with the nasal cavity proper are poorly known (Coombs and
Maryanska, 1990).
Although similar paranasal cavities have not been reported
for nodosaurid ankylosaurs, it is not clear if sufficient material
providing the necessary views is available to regard this as a
positive absence. The frequently figured cross-section of Edmontonia
longiceps (AMNH 3076; Coombs, 1971, 1978; Norman,
1985; Coombs and Maryanska, 1990) indeed shows a single,
large cavity on each side within the snout (Fig. 19B). However,
this cavity is partially subdivided by a dorsally projecting
maxillary ridge that housed the tooth roots (Coombs, 1971).
Since the cross-section (produced by a natural break) passes
through that portion of the snout in which the antorbital cavity
is lodged in other ornithischians, the possibility remains that
the maxillary ridge is a septum between nasal cavity proper and
antorbital cavity, the snout having broken transversely at the
position of the ostium (which would be dorsal to the septum).
The point is that nodosaurids may indeed have paranasal cavities
similar to, although simpler than, those in ankylosaurids.
Dinosauria: Ceratopsia-There are three major groups of
ceratopsian ornithischians, all presenting different conformations
of the snout. In the basal ceratopsian Psittacosaurus spp.
(Sereno, 1986), the external antorbital fenestra is probably absent.
Earlier reports of the presence of a homologue of the maxillary
antorbital fossa (Sereno, 1987; Sereno and Chao, 1988;
Sereno et a]., 1988) were later corrected (Sereno, 1990). Most
specimens of Psittacosaurus spp. have an unossified gap between
the lacrimal and premaxilla opening into a cavity in
which the nasolacrimal canal opens (Sereno, 1987, 1990; Sereno
et al., 1988). It is possible that this gap represents the remnants
of an external antorbital fenestra, with the expansive caudodorsal
lamina of the premaxilla restricting the size of the
opening as it overlaps the lacrimal. The course of the nasolacrimal
duct (Sereno, 1987; Sereno et al., 1988) tends to support
this assessment. In juveniles of Psittacosaurus mongoliensis
(AMNH 6535, 6536; see also Coombs, 1982; Sereno, 1987),
this gap (perhaps best viewed as a fontanelle) is somewhat larger,
although preservational artifact may increase its apparent
size. Arguing against this notion is the observation of a specimen
of this species in which the gap is fully within the lacrimal
bone (Sereno, 1987), which would be unlike an external antorbital
fenestra. Thus, it is probably judicious to follow Sereno
(1990) in regarding the external antorbital fenestra as absent.
The snouts of basal neoceratopsians ("protoceratopsians")
are well known from many specimens (Brown and Schlaikjer,
1940; Maryanska and Osm6lska, 1975; Osmolska, 1986). In
Protoceratops andrewsi (many AMNH specimens), Bagaceratops
rozhdestvenskyi (Maryanska and Osmolska, 1975), and
Leptoceratops gracilis (Sternberg, 195 l), there is a small internal
antorbital fenestra between the maxilla, lacrimal, and usually
the jugal (Fig. 20A, B). The internal fenestra is relatively
larger in young individuals (e.g., Protoceratops andrewsi,
AMNH 6421). This aperture clearly communicates solely with
the nasal cavity directly opposite the caudal portion of the choana
(Osmolska, 1986). In most specimens, there is a wellmarked
antorbital fossa excavated into the maxilla, lacrimal,
jugal, and, in Bagaceratops rozhdestvenskyi, the nasal. In most
specimens of Protoceratops andrewsi, the antorbital cavity undercuts
the margins of the external antorbital fenestra so that
there are shallow lacrimal and jugal recesses and much deeper
maxillary recesses; in many larger skulls (AMNH 6433, 6429,
6414, 6466) the rostral portion of the fossa is not deeply excavated
into the surface (Fig. 20A, B). In B. rozhdestvenskyi
and P. andrewsi, an accessory cavity is present within the body
of the maxilla (Maryanska and Osmblska, 1975; Osmolska,
1986; see below).
The course of the nasolacrimal duct is unknown, and in fact,
despite excellent material of Protoceratops andrewsi (AMNH
6429), no evidence of the nasolacrimal canal could be discovered.
Sternberg (1951) also could not locate a canal in Leptoceratops
gracilis. It should be noted that Bagaceratops rozhdestvenskyi
has an additional opening in the snout rostral to the
antorbital cavity, between maxilla and premaxilla (Maryanska
and Osmblska, 1975); this structure remains poorly understood,
but it appears to have little to do with the antorbital cavity.
In Ceratopsidae, the specified osteological correlates are easily
interpreted, although the antorbital cavity is apparently very
small. The internal antorbital fenestra is usually little more than
an oval foramen situated between maxilla and lacrimal with
varying contribution from the nasal and jugal (Dodson and Currie,
1990). It opens medially into the nasal cavity opposite the
caudal portion of the choana; the palatine sends a lateral process
to form the caudomedial border of the internal fenestra (Hatcher
et al., 1907). Although there is sometimes a small maxillary