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26 SOCIETY OF VERTEBRATE PALEONTOLOGY, MEMOIR 3
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rnax pal
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promax max
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FIGURE 16. Antorbital cavities in left lateral view of A, Hesperornis
regalis and B, Archaeopteryx lithographica. (Modified from Witmer,
1990.)
the caudal portion of the internal antorbital fenestra. As in most
other ornithurine birds, the nasolacrimal duct of hesperornithids
grooved the lateral surface of the lacrimal on its way through
the external antorbital fenestra.
Archaeopteryx lithographica has a more primitive antorbital
cavity than perhaps any known bird (Fig. 16B) and provides an
important transitional link to more basal forms. In particular, it
retains (unique among birds) the dorsal portion of the nasal
process of the maxilla (Cracraft, 1986; Witmer, 1990). In A.
lithographica, this portion of the maxilla (the "ascending ,ramus"
of other archosaurs) is recessed and fenestrated as in
many other theropods, enclosing a large portion of the antorbital
cavity. Because part of the maxilla is recessed, the external
antorbital fenestra is formed by the maxilla, lacrimal, and nasal.
The rostral margin of the internal antorbital fenestra is formed
by the medial lamina of the ascending ramus. There is probably
not much of an antorbital fossa on the lacrimal except perhaps
ventrally (BMNH 37001), but the recessed part of the ascending
ramus of the maxilla forms a broad antorbital fossa. As mentioned,
the ascending ramus has two fenestrae (Wellnhofer,
1974), the rostral one (i.e., the promaxillary fenestra) apparently
being associated with a small chamber within the maxilla opening
caudally into the antorbital cavity (Witmer, 1990). The fenestrae
are the "subsidiary antorbital" or "maxillary" fenestrae
of other authors and are widely distributed among theropod
dinosaurs. They cannot be studied adequately in the crushed
material referable to A. lithographica, but are given more attention
in the discussion of accessory cavities below.
New discoveries of palatal elements of Archaeopteryx sp.
(cast of Solenhofer Aktien-Verein specimen; see also Elzanowski
and Wellnhofer, 1996; Paul, 1996) confirm that the choanae
were directly opposite the internal antorbital fenestra (Witmer
and Martin, 1987). The vertical shaft of the lacrimal
(BMNH 37001) is pierced by a foramen that almost certainly
is an opening for the nasolacrimal duct, suggesting that the duct
passed through the dorsal portion of the antorbital cavity over
the air sac without otherwise being enclosed in a bony canal.
Among non-avian theropods, many of the correlates are clear
(Fig. 14) and require little discussion. For example, the choana
is always directly opposite the internal antorbital fenestra. Similarly,
the internal antorbital fenestra opens medially into the
nasal cavity and laterally into a space excavated into the surrounding
bones. In fact, the antorbital fossa in most non-avian
theropods occupies most of the snout and, in some forms, much
of the skull (about 45-55% of total skull length in Coelophysis
bauri and Proceratosaurus bradleyi [BMNH R48601). The
maxillae of all theropods (except Herrerasaurus ischigualastensis
and some abelisaurids) have extensive antorbital fossae
on their lateral surfaces. In most cases, the fossa extends far
ventrally below the margin of the internal antorbital fenestra,
occasionally approaching the labial edge of the bone (e.g., Ceratosaurus
nasicornis, USNM 4735; Dilophosaurus wetherilli,
UCMP 37303, 77270; Coelophysis bauri, many specimens, Fig.
14; Allosaurus fragilis, UUVP 5427, BYU 5126, USNM 4734;
Ornitholestes hermanni, AMNH 619; Proceratosaurus
bradleyi, BMNH R4860; many others). In other clades, however,
the fossa extends only slightly below the internal fenestra
(e.g., some tyrannosaurids) or essentially not at all (e.g., abelisaurids
[Bonaparte et al., 1990; Bonaparte, 1991 a], Carcharodontosaurus
saharicus [SGM-Din 1; see Sereno et al., 19961,
troodontids [Saurornithoides mongoliensis, AMNH 65 16, Troodon
formosus, CMN 12392; see also Osmdlska and Barsbold,
19901). In virtually all non-avian theropods, the maxillary antorbital
fossa extends onto the ascending ramus (Fig. 14). Most
theropods also have portions of the antorbital fossa extending
onto the lacrimal bone, with a common pattern being ventrolateral
and dorsolateral fossae separated by a sculptured (subcutaneous)
area (e.g., Coelophysis bauri, CM 31374, Fig. 14;
Ceratosaurus nasicornis, USNM 4735; Allosaurus fragilis,
UUVP 2133; most tyrannosaurids). Often the ventrolateral lacrimal
fossa extends onto the jugal, such that there is a prorninent
recess caudoventrolateral to the internal antorbital fenestra.
Whereas in most theropods the lacrimal and maxillae exclude
the nasal from the antorbital fossa, the nasal bone enters into
the fossa in a few taxa (e.g., Monolophosaurus jiangi [Zhao
and Cume, 19941 and Allosauroidea [see Cume and Zhao,
1994a; Sereno et al., 19961). Associated with the antorbital fossae
in many non-avian theropods are various foramina and accessory
cavities within the facial bones (see below).
The course of the nasolacrimal duct is another landmark for
the main paranasal sinus of extant archosaurs, in which it passes
dorsally over the major part of the sinus, then becoming medial
to the sinus as it approaches the choana. In non-avian theropods,
the bony nasolacrimal canal passes wholly within the lacrimal
bone. Its orbital aperture is usually (if not always) single,
but there is considerable variation in the subsequent course of
the duct. For example, in Dromiceiomimus brevitertius (CMN
12228) and Troodon formosus (RTMP 82.19.23; Cume, 1985),
the nasolacrimal canal is long and runs for some distance within
the rostral ramus of the lacrimal; its rostral (nasal) aperture
opens medially. In Allosaurus fragilis (UUVP 2133) and Deinonychus
antirrhopus (MOR 747; see Witmer and Maxwell,
1996), the short canal opens into the lacrimal recess and the
epithelial duct must have continued rostrally through the inter-