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NUMBER 89 319
thids (Figure 5), oviraptorids (Barsbold, 1983b, fig. 13), at least
in Garudimimus among the omithomimosaurs (Barsbold and
Osmolska, 1990, fig. 8.3.D), the paleognaths (Muller, 1963;
pers. obs.), the remaining neognaths, and probably Gobipteryx.
MANDIBULAR SYMPHYSIS.—The symphysis is unfused in
Archaeopteryx, although the rostral tips of the rami are connected
very tightly. Among the theropods, the ends of the mandibular
rami are fused only in the oviraptorosaurs and are fused
even in the smallest caenagnathid specimens (Currie et al.,
1993), but they remain unfused in the oviraptorid embryo
(Norell et al., 1994), suggesting that fusion occurred at or soon
after hatching, as in Gobipteryx (Elzanowski, 1981). The symphysis
is fused in Confuciusomis, which combines an Archaeopteryx-like
postcranial skeleton with a Gobipteryx-like skull
(Hou et al., 1995), Gobipteryx (Elzanowski, 1977), and other
omithurines except for the odontognaths (Marsh, 1880), teratorns
(Campbell and Tonni, 1981), and pseudodontoms (Odontopterygia)
(Zusi and Warheit, 1992).
The lack of a symphysis in the odontognaths is most probably
secondary because in the hesperomithids and possibly in
Ichthyornis, the tips of the rami were connected by a predentary
bone (Martin, 1987), which is definitely a derived character
because no other bird or theropod has it. The dentary of
birds arises from two centers of ossification (Nemeschkal,
1983b), and the predentary bone most probably ossified from
the rostral (mentomandibular) center. Consequently, this bone
probably represents the co-ossified symphyseal part that became
separated from the remainder of the mandible.
The pseudodontoms are highly specialized, fish-eating neognathous
birds related to the pelecaniforms, which makes the
lack of mandibular symphysis in these birds unquestionably
secondary.
As of now, there is no evidence for multiple origins of the
fused symphysis in birds. It may have evolved only once in the
omithurines, and its presence could be another synapomorphy
of the oviraptorosaurs and omithurines.
INTRARAMAL JOINT.—This joint includes the articulations
between the dentary and surangular and between the splenial
and angular. It permits mediolateral mobility within each ramus
of the mandible. The majority of the theropods, including
the dromaeosaurids, have an intraramal joint. It is absent in the
omithomimosaurs, Erlikosaurus, oviraptorosaurs, and primitive
birds, including Archaeopteryx, Gobipteryx, and probably
Confuciusomis. In more advanced birds, there are at least two
types of intraramal joints: one in the pelecaniforms (including
pseudodontoms) and another in the odontognaths. They are
made of different components, which indicates that they
evolved independently of each other (Zusi and Warheit, 1992).
Gingerich (1973) proposed that the intraramal joints in Hesperomis
and Ichthyornis have been inherited from the theropods
and thus represent yet another theropod/avian synapomorphy.
Consequently, the similarity between Ichthyornis and
hesperomithids would be symplesiomorphic. This, however, is
inconsistent with the intraramal joint being absent in Archaeopteryx,
Confuciusomis, and Gobipteryx (see also Feduccia,
1996:155). In addition, the two kinds of intraramal joints in
birds are more similar to each other (Gingerich, 1972, fig. 1;
Zusi and Warheit, 1992, fig. 7) than either of them is to the
joint in the dromaeosaurids (Currie, 1995, fig. 7). In the odontognaths,
the hinge is formed primarily by the splenial and angular,
and these bones articulate at an angle of-45° to the long
axis of the ramus. In the dromaeosaurids the hinge, reinforced
by bony knobs, is primarily between the dentary and surangular,
whereas the splenial and angular seem to form a gliding articulation
that is oriented much more horizontally than the splenio-angular
hinge of the odontognaths. In all probability, birds
started their evolution with a rigid mandible, and detailed similarity
of the intraramal joints in Ichthyornis and hesperomithids
is synapomorphic.
JUGAL BAR.—Confuciusomis and probably all the ornithurine
birds have a thin, rod-like jugal bar that is formed in substantial
part, and in some neognaths exclusively, of the quadratojugal.
In the oviraptorids and Avimimus (Kurzanov, 1987),
the bar is thin, as it is in the omithurines. The slender jugal bar
stands out in an otherwise massive oviraptorid skull adapted
for durophagy.
In Archaeopteryx and the remaining known theropods, the
jugal bar is a robust slat. It is formed almost exclusively of the
jugal in most of the theropods. In Archaeopteryx, the quadratojugal
may have extended far rostrally on the medial side of the
jugal (Elzanowski and Wellnhofer, 1996, fig. 7).
The postorbital process of the jugal is in the terminal caudal
position, and the infratemporal fossa is reduced in the omithomimosaurs,
the troodontids, and Archaeopteryx. Erlikosaurus
is unique among the theropods in having two ascending processes
of the jugal: a postorbital process one-fourth the length
from the caudal end, and a terminal process overlying the quadrate
and approaching the squamosal. The latter corresponds in
position to the postorbital process of the omithomimosaurs and
Archaeopteryx. This unique jugal morphology raises the possibility
of a secondary enlargement of the infratemporal fossa,
accompanied by the division of the initially terminal postorbital
process. Although the rostral position of the postorbital bar
in the oviraptorids looks like a symplesiomorphy with the majority
of theropods, it is conceivable that this position is secondary
and that the postorbital process of the jugal corresponds
to the rostral, possibly secondary, process in Erlikosaurus.
INTERDENTAL PLATES AND TEETH.—The presence of interdental
plates is a primitive archosaurian character (Elzanowski
and Wellnhofer, 1993). Among the theropods, inderdental
plates are known to be absent in troodontids (Currie, 1987), Archaeomithoides
(Elzanowski and Wellnhofer, 1993), Baryonyx
and Spinosaurus (Buffetaut, 1992), and omithomimosaurs
(Perez-Moreno et al., 1994). Separate interdental plates are
present in juvenile dromaeosaurids (Norell et al., 1994) and
seem to be replaced by porous interdental bone in adults (Currie,
1987).
The discovery of interdental plates in Archaeopteryx (Wellnhofer,
1993) makes it clear that their absence in Hesperomis
and Ichthyornis is secondary, as probably are other aspects of