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320 SMITHSONIAN CONTRIBUTIONS TO PALEOBIOLOGY
their tooth implantation, such as the expanded dental roots and
the equal height of the lingual and buccal margins of the dentary
(Elzanowski and Wellnhofer, 1995). The teeth of Ichthyornis
are usually anchored in separate alveoli, whereas those of
the hesperomithids are in a common groove; however, Martin
and Stewart (1977) described a mandible of Ichthyornis with
teeth in a groove.
Avian teeth do not show any close similarity to any known
theropod teeth except for the early juvenile teeth of Archaeornithoides
(Elzanowski and Wellnhofer, 1993). The teeth of Archaeopteryx
and Ichthyornis are similar in having the crowns
divided into a straight base and a recurved tip. On the other
hand, the teeth of Ichthyornis were reported to have the roots
distinctly expanded, as in the hesperomithids (Martin and
Stewart, 1977). Unfortunately, the radiographs on which this
observation is based have never been published. Dental roots
are not expanded in Archaeopteryx (Howgate, 1984; Elzanowski
and Wellnhofer, 1996). 1
Entirely toothless jaws evolved independently in the oviraptorosaurs,
advanced omithomimosaurs, Confuciusomis and
Gobipteryx (these two genera were included in the Enantiornithes
by Hou et al., 1995), and the advanced neomithines. An
edentulous premaxilla coexists with a dentigerous maxilla and
dentary in the therizinosauroids and hesperomithids. It is clear
that the reduction of teeth occurred several times independently
within the analyzed set of taxa.
Discussion
Some of the avian characters, such as the toothless beaks of
the oviraptosaurs and advanced omithomimosaurs, are certainly
convergent and evolved more than once within the set of
taxa under comparison. Reduction of teeth is a recurrent theme
in vertebrate evolution.
There is, however, no reason to expect a pervasive convergence
on birds in the cranial morphology of the omithomimosaurs,
therizinosauroids, and oviraptorosaurs because their
skulls are unlike any avian skull. This is what one could expect
as a result of turning a primitive, perhaps Archaeopteryx-like
cranial morphology into several highly specialized kinds of jaw
apparatus of the gigantic descendants of the ancestors of birds.
In terms of jaw function, the oviraptorids are comparable to the
dicynodonts (Osmolska, 1976), and the therizinosauroids are
convergent on the omithischians in having cheeks (Clark et al.,
1994). It seems reasonable, therefore, to suspect that the unique
cranial similarities of birds, oviraptorosaurs, therizinosauroids,
and omithomimosaurs (Table 1, characters 1-6) are synapo-
1 Editor's Note: This is at variance with Martin and Stewart, elsewhere in this
volume, who consider previously published photographs and descriptions to
indicate that the teeth of Archaeopteryx do have expanded roots.
morphic and thus indicative of the monophyly of a clade composed
of these taxa and probably troodontids (Figure 6). This
would agree with the phylogeny reconstmcted from 20 cranial
and 39 postcranial characters by Russell and Dong (1993), who
subdivided the tetanurans into two groups: one that included
the dromaeosaurs and camosaurs and one that included the ornithomimosaurs,
troodonts, therizinosauroids, and oviraptorosaurs.
Most intriguing are four characters (Table 1, characters
10-13) that are shared by the oviraptorosaurs and the ornithurine
birds but are absent in Archaeopteryx. These suggest that
the oviraptorosaurs branched off after Archaeopteryx and thus
represent the earliest known flightless birds. Except for the
elongate forelimbs (which become shortened in all flightless
forms), the postcranial skeleton of Archaeopteryx does not
have any avian traits that would be absent in the oviraptorids
(Barsbold 1983a, 1983b). Therefore, if flightlessness had
evolved at a stage of avian evolution close to Archaeopteryx,
this would be extremely difficult to distinguish from the primary
flightlessness of the theropods.
Relationships of the oviraptorosaurs have been enigmatic
ever since their discovery (Osbom, 1924). The only consensus
reached in recent phylogenetic reconstmctions is that the oviraptorosaurs
belong in the Coelurosauria, a major clade that
gave rise to birds, but their placement within that clade varies
considerably (Gauthier, 1986; Barsbold et al., 1990; Holtz,
1996). Evidence for their affinities comes almost exclusively
from the postcranial skeleton because the oviraptorid skull is
difficult to compare with the skull of any other group of theropods.
In contrast, even the highly specialized, edentulous skulls
of omithomimosaurs are still clearly identifiable as theropodan.
The two cranial characters used by Gauthier (1986) to define
the Coelurosauria, namely, the pterygopalatine fenestra and
ventral pocket in the ectopterygoid, are absent in the oviraptorids
(as well as in the omithurines).
Several similarities of the jaws of Hesperomithidae and Ichthyomithidae
prove likely to be synapomorphies when analyzed
against the plesiomorphic background provided by the
theropods and Archaeopteryx. Wetmore (1930) combined the
Hesperornifhiformes and Ichthyornithiformes in the superorder
Odontognathae, but monophyly of this taxon has never been
explicitly suggested because most of their similarities were
thought to be primitive. A closer relationship between these
two orders is now suggested by the lack of mandibular symphysis,
probably due in both taxa to the separation of the tip of
the mandible as a predentary bone; the presence and detailed
similarities of the intraramal joint; the absence of interdental
plates, which is probably correlated with the lingual alveolar
margin being flush with the buccal margin; and probably the
expansions of the dental roots (fide Martin and Stewart, 1977)
and the straight quadrate.