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318 SMITHSONIAN CONTRIBUTIONS TO PALEOBIOLOGY
In the troodontids (Currie, 1985; Currie and Zhao, 1993) the
basipterygoid processes are strong but are directed caudolaterally
rather than rostrolaterally as in other theropods. This precludes
a typical theropod basipterygoid articulation, and their
facets are strongly concave, which suggests a basal process on
the pterygoid (the caudal part of which is unknown in the troodontids).
In the dromaeosaurids and most of the remaining
theropods, the pterygoids have sockets that receive prominent
basipterygoid processes of the sphenoid.
QUADRATE.—In the dromaeosaurids and most of the remaining
theropods, the caudal profile of the quadrate is straight, and
the quadrate socket in the squamosal opens more or less ventrally.
In contrast, the quadrate head is bent backward in the
oviraptorids (Maryariska and Osmolska, 1997), Erlikosaurus,
omithomimosaurs, troodontids, Gobipteryx, and the majority
of other omithurines except for Hesperomithidae, Ichthyornis,
and some modem birds. In Archaeopteryx the caudal bend of
the quadrate head may be partly obscured by the position in
which the quadrate is preserved; it is exposed in caudal aspect
in the seventh skeleton and in rostral aspect in the fifth skeleton.
The oviraptoid quadrate alone agrees with that of the omithurines
in having an otic capitulum for the articulation with
the braincase, the pterygoid articular surface approaching the
medial mandibular condyle, and a distinct (but shallow)
quadratojugal cotyla (Maryariska and Osmolska, 1997).
CORONOID.—The coronoid bone is absent in the oviraptorosaurs
(Figure 5), omithomimosaurs, Erlikosaurus, and all adult
birds, including Archaeopteryx and Gobipteryx. It may have
been incorporated into the prearticular (Nemeschkal, 1983a),
and a tiny splint of bone just rostral to the tip of the prearticular
in the hatchling Golden Eagle {Aquila chrysaetos (Linnaeus))
probably is a vestige of the coronoid (Jollie, 1957).
ARTICULAR AND PREARTICULAR.—The articular of ornifhurine
birds (including Gobipteryx), but not Archaeopteryx,
co-ossifies with the prearticular, surangular, and angular at various,
mostly posthatching, stages of development. The articular
and surangular are co-ossified in the caenagnathids, Erlikosaurus,
and all other adult omithurines. The articular is co-ossified
with the angular and surangular in Avimimus, in which the
prearticular is not preserved (Kurzanov, 1987). The articular is
separated by sutures from the adjacent bones in the omithomimosaurs,
oviraptorids, and other theropods, including dromaeosaurids
and Allosaurus.
The articular surface for the quadrate is expanded both medially
and laterally beyond the ramus in ornithurine birds. The lateral
expansion is coextensive with the small lateral process that
bears it. By contrast, the medial expansion covers only the basal
part of the prominent medial process, which provides an area of
attachment for the pterygoideus muscle. In Gobipteryx, however,
there is no connection between the medial expansion of
the articular surface and the medial process, which may or may
not be due to damage. There is no evidence of any distinctive
projections of the articular surface in Archaeopteryx or in the
omithomimosauria. The articular surface is strongly expanded
laterally but not medially in Erlikosaurus and is expanded both
medially and laterally in the oviraptorids and caenagnathids.
The two projections in the oviraptorosaurs are similar to those
in Gobipteryx (Elzanowski, 1974, fig. XXXIII/2).
Aside from being reduced in a few neognaths (such as the
phasianids), the prearticular shows two fairly distinctive morphologies.
It turns dorsally and expands into an ascending
blade in Erlikosaurus, the majority of theropods (including the
dromaeosaurids), Archaeopteryx, Hesperomis (pers. obs.), and
some neognaths (e.g., Spheniscidae, Laridae). The prearticular
continues far rostrally as a straight bony rod in the caenagna-
FlGURE 5.—Mandible of Chirostenotes pergracilis (-Caenagnathus collinsi R.M. Sternberg), CMN 8776 in
medial view. The medial aspect of this mandible has been hitherto described only by Sternberg (1940, fig. 2).
Sternberg's illustration showed a well-defined caudal outline of the dorsomedial process of the dentary, whereas
in reality this process seems to be fused to the surangular. Absent from the mandible is the splenial, which may
have fallen off, as frequently happens in birds, but it also is possible that it was reduced because the mandible is
unusual in having a rostral extension of the angular that reaches the symphysis. The prearticular extends to the tip
of the retroarticular process and covers the articular, which is fused to the surangular but not to the prearticular.
(an=angular, d=ventral ramus of dentary, dv=ventral process of dorsal ramus of dentary, pa=prearticular,
pc=coronoid process, pm=medial process, pr=retroarticular process, rl=lateral ridge, rm=medial ridge,
sa=surangular.)