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32 SOCIETY OF VERTEBRATE PALEONTOLOGY, MEMOIR 3<br />
lac \ fen antorb int<br />
fos antorb max<br />
I<br />
- 2 ern<br />
FIGURE 20. Protoceratopsians. A, stereophotographs <strong>of</strong> snout <strong>of</strong> Prorocerafops andrewsi (AMNH 6433) in right lateral view. B, interpretive<br />
drawing <strong>of</strong> A. C, schematic transverse section <strong>of</strong> the snout <strong>of</strong> Bagaceratops rozhdesmenskyi showing the intramaxillary sinus. Left side <strong>of</strong> figure<br />
depicts a more rostral section than the right side which is more caudal. D, more rostral transverse section <strong>of</strong> the same species. (C,D modified<br />
after OsmBlska, 1986).<br />
antorbital fossa, the external antorbital fenestra is not usually<br />
well marked, and the fenestra is sometimes completely closed.<br />
Internally, the antorbital cavity passes into a cavity formed between<br />
maxilla and palatine and through which passed the maxillary<br />
neurovasculature (well displayed in a disarticulated skull<br />
referable to Chasmosaurus belli, BMNH R4948). The jugal<br />
walls this cavity caudolaterally (Lehman, 1989), such that the<br />
whole area, although highly transformed, exhibits the same topographic<br />
relations as in more basal ornithischians. The<br />
course-indeed, even the presence<strong>of</strong> the nasolacrimal canal<br />
is not discernible in many specimens. Lehman (1989) described<br />
one for Chasmosaurus mariscalensis, but it is noted as absent<br />
in Triceratops horridus by Forster (1990) and Centrosaurinae<br />
by Sampson (1993).<br />
Dinosauria: Pachycephalosauria-Most <strong>of</strong> the relevant<br />
data on Pachycephalosauria come from Maryanska and Osm6lska's<br />
(1974) study <strong>of</strong> Prenocephale prenes. P. prenes is the<br />
only known pachycephalosaur that retains an external antorbital<br />
fenestra, which forms a small opening between maxilla and<br />
lacrimal; there is no external fossa surrounding the fenestra.<br />
The external fenestra leads internally into a large antorbital cavity<br />
(the "intramaxillary sinus" <strong>of</strong> Maryanska and Osm6lska,<br />
1974) that extends within the length <strong>of</strong> the maxilla and is<br />
bounded caudally by lateral and medial laminae <strong>of</strong> the lacrimal.<br />
The cavity opens medially into the nasal cavity directly opposite<br />
the choana. The nasolacrimal canal pierces the lacrimal,<br />
passing dorsomedially through the antorbital cavity. Furthermore,<br />
the maxillary neurovasculature enters the cavity caudoventrally.<br />
Thus, this structure displays the specified osteological<br />
correlates. Although no other known pach~cephalosaurs retain<br />
an external antorbital fenestra, Maryanska and Osm6lska<br />
(1974) noted that the cavity was found in other members <strong>of</strong> the<br />
clade. Based on the pattern <strong>of</strong> crushing, Sues and Galton (1987)<br />
suggested that the sinus was present also in Stegoceras validum.<br />
These other forms, therefore, carry to its extreme the ornithischian<br />
trend <strong>of</strong> enclosure <strong>of</strong> the antorbital cavity (see below).<br />
Pterosauria-All pterosaurs retain an antorbital cavity (Fig.<br />
21), but apparently in all Pterodactyloidea (Bennett, 1994) the<br />
ascending ramus <strong>of</strong> the maxilla has been lost and the reciprocal,<br />
maxillary (subnarial) process <strong>of</strong> the nasal has been reduced so<br />
that the naris and .antorbital cavity have become confluent,<br />
forming the nasoantorbital fenestra. Among "rhamphorhynchoids"<br />
(generally assumed to be a paraphyletic grouping; see<br />
Padian, 1985), there is usually little or no development <strong>of</strong> an<br />
antorbital fossa so that the internal and external antorbital fenestrae<br />
have the same borders (e.g., Campylognathoides liasicus,<br />
CM 11424; Rhamphorhynchus longiceps, CM 11428;<br />
Rhamphorhynchus muensteri, CM 11431; see also Wellnh<strong>of</strong>er,<br />
1978; Wild, 1978). The borders <strong>of</strong> the fenestra vary but usually<br />
include the maxilla, lacrimal, jugal, and sometimes nasal. In<br />
probably all pterodactyloids (Bennett. 1991, in press), rostroventrally<br />
directed processes from each nasal fuse, forming a