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would seem unlikely at the outset because the enantiornithine
birds of China have Hesperornis-like teeth, almost certainly establishing
this structure as primitive for birds. Examination of
the London specimen of Archaeopteryx shows typical avian
structure, with a waisted crown, expanded root, and oval replacement
pits (Figure 3A-C). Elzanowski and Wellnhofer
(1995:42) deny the presence of expanded roots in Archaeopteryx
in spite of the fact that such roots are clearly shown in
Wellnhofer (1988, pl. 8: figs. 2, 4; 1993, pl. 6: figs. 1, 3, 5). It
also is ironic that the first mention of expanded roots in bird
teeth dates back to the first description of teeth in Archaeopteryx
by Evans (1865), and this was later confirmed by Edmund
(1960). There is an excellent oblique photograph (Wellnhofer,
1993, pl. 5: fig. 9) of the seventh specimen of
Archaeopteryx showing the waisted crown and expanded root
typical of other birds and the so-called interdental plates crossing
as tooth septa. Although the features of bird teeth may be
most easily seen in large isolated specimens from Hesperornis
and Parahesperornis, it is clear that all known bird teeth are
closely similar to each other and that the teeth of Archaeopteryx
are not atypical or especially primitive. The claim for interdental
plates (Elzanowski and Wellnhofer, 1996) in the seventh
skeletal specimen is based on the labial margin of the jaw
being higher than the lingual margin and exposing distinct alveolar
septa (interdental plates?) between the tooth sites. In
fact, this condition is better displayed in the London maxillary
(Howgate, 1984), where it has been interpreted as a tooth socket
(Martin, 1991). Because the alveolar bone of the sockets and
the attachment bone of the interdental plates are ultimately derived
from the same source, we must carefully describe what is
meant by interdental plates versus sockets. When interdental
plates are present, they generally expose the replacement teeth,
and most of the length of the root is surrounded on the lingual
side by the interdental plates. The jaws of the seventh specimen
of Archaeopteryx are spread and compressed so that we get a
slightly oblique view of the jaws on the slab (see Wellnhofer,
1993, pl. 4: fig. 1). We are indebted to Wellnhofer (1993) for
excellent photographs that clearly show the crown-root juncture
on the teeth of the seventh example and show that the constriction
at the base of the crown is almost at the lingual edge of
the mandible (Figure 3E), so that the replacement teeth are
mostly hidden by the side of the dentary as in other birds.
When we look at typical interdental plates (Figure lG), we
see that not only the socket but also part of lingual side of the
jaw is produced by the interdental plates, and that there is a distinct
groove separating the individual plates, terminating in the
"special foramina" and the replacing teeth. The replacing teeth
lie to the lingual side of the adult tooth as shown by the replacing
tooth in Troodon (Figure IF). These are not the relation­
ships shown in the London Archaeopteryx (Figure 3A-C) or in
the seventh specimen (Figure 3E). The intersepta of the tooth
sockets of the London maxilla closely resemble a similar view
of an alligator maxilla (Figure 3F), as well as the sockets of Archaeopteryx
(Figure 3E) so well photographed by Wellnhofer
(1993). A close examination of Wellnhofer's photographs also
shows the intersepta widening again as they come to the labial
edge, as expected in a dorsal view of the socket. If the view
were entirely medial, we would not expect to see this widening,
even if these were interdental plates (see Figure lG).
The condition in coelurosaurs is not as clear as it is in camosaurs.
Compsognathus is reported by Ostrom (1978) to have
small interdental plates. Dromaeosaurs were not thought to
have interdental plates (Colbert and Russell, 1969). According
to Currie (1995), dromaeosaurs have fairly typical interdental
plates forming much of the lingual side of the jaw below the
sockets except that the grooves fuse across, forming a solid
wall. This should indicate a modified tooth replacement, and
indeed it appears that the tooth family may be thrown into diagonal
lines so that replacing teeth are both lingual and posterior,
as, for example, in the overlapping replacement tooth in the ramus
of Deinonychus illustrated by Ostrom (1969). This is not
bird-like, nor is the covering of the interdental plates by special
bones (supradentary) in Allosaurus and Tyrannosaurus (intercoronoid
of Brown and Schlaikjer, 1940). It is clear that coelurosaur
teeth are very similar to the teeth of camosaurs and do
not show the specialized type of tooth replacement found in
birds (Figure 2A-D).
Conclusion
The argument that birds are related to dinosaurs is now most
often restated that birds are dinosaurs. If this is the case, we
would expect their anatomical structures to maintain similarity
under a very rigorous analysis. We see that this is not tme for
almost any aspect of tooth form, implantation, or replacement.
The tooth structures identified as interdental plates in Archaeopteryx
by Wellnhofer (1993) do not agree in detail with those
structures in dinosaurs and can be closely duplicated by crocodilians.
We now know from the abundant Chinese enantiornithine
material that the tooth form of birds is similar in all
known groups of birds and must have been established at least
by the Jurassic. Crocodilians and birds form the inside walls of
their tooth-bearing bones differently from dinosaurs and have a
different mode of tooth replacement. Their common ancestor
with dinosaurs may not have been "thecodont" in the descriptive
sense of that word. Crocodilians have derived features that
prevent them from being ancestral to birds, but a sister-group
relationship is still possible.