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298 SMITHSONIAN CONTRIBUTIONS TO PALEOBIOLOGY
Bird teeth do contain circular or oval replacement pits that
are closed on the bottom, and the developing crown also must
tilt as it enters the root of its predecessor (Figures 2E, 3A-D).
They also have root cementum (Schmidt and Keil, 1958), and
therefore their teeth are attached with periodontal ligaments.
Peyer (1968) reported the presence of tooth cementum in
crocodiles among modem reptiles and among ichthyosaurs in
fossil reptiles, but in ichthyosaurs it is developed only in the
geologically younger forms (Peyer, 1968:146). Ichthyosaurs
also developed expanded roots, and Peyer (1968:146) suggested
that this was "to offer the connective tissue fibers an adequate
surface for attachment." It seems likely that in crocodilians,
birds, and ichthyosaurs, the expanded roots are related to
a combination of problems resulting from teeth set in a groove.
We have not clearly identified the crocodilian pattern of thecodonty
in any other diapsid reptile, but we would expect that
the peculiar mode of tooth replacement would accompany it, if
it does occur elsewhere.
Currie (1987) made an effort to identify bird-like characteristics
in the jaws and teeth of troodontid theropods but figured a
cross section (reproduced herein as Figure IF) showing a typical
dinosaurian lingual replacement pattern and interdental
plates (Currie, 1987, figs. 1, 3). The crowns of the teeth are
wider than the roots and are widest at the point that they join
the roots (not waisted). The teeth are heavily serrated (Figure
2B). In other words, they do not show a single feature thought
to characterize bird teeth (Martin et al., 1980). Currie and Zhao
(1993) published a drawing of an undetermined dinosaur tooth
(?dromaeosaurid) thought to show an oval replacement pit;
however, this tooth was an isolated find, was poorly figured,
and cannot be relocated. No other dinosaurian taxa with
bird-like teeth have been identified in the 20 years since the
unique features of bird teeth were first described (Martin et al.,
1980). Currie and Zhao (1993:2245) also suggested that because
bird teeth tend to drift out of the jaws after death, they
could not have been attached by cementum. This must be based
on a misunderstanding because the decay of the periodental ligaments
would release the teeth of birds and young crocodilians,
which would be found as relatively intact teeth with roots attached,
as noted by Currie and Zhao (1993). This happens
much more rarely with dinosaurs, where the teeth are fixed by
attachment bone. It also should be pointed out that theropod dinosaurs
have relatively much more room for the lingual tooth
family than is found in either birds or crocodilians (contrary to
Currie and Zhao, 1993:2245). The ornithomimid dinosaur
Mononykus, considered to be a bird by some (Perle et al.,
1993), has teeth (Figure 2A) resembling those of the ornithomimid
Pelecanimimus, not those of birds.
Elzanowski and Wellnhofer (1996) took the opposite tack by
attempting to show that the jaws and teeth of Archaeopteryx
are like dinosaurs rather than like other toothed birds. This
FIGURE 3.—A-C, Lingual views of the premaxillary and maxillary teeth of
Archaeopteryx lithographica von Meyer, London specimen (BMNH 37001):
A, left premaxilla and right maxilla; B, maxilla and isolated tooth; c, isolated
tooth (from right premaxilla?); D, Parahesperornis alexi, left lower tooth
(from the holotype); E, drawing taken from photograph of a tooth of the seventh
specimen of Archaeopteryx (in Wellnhofer, 1993, pl. 6: fig. 3), showing
similarity to sockets in the London maxillary; F, right, lingual view of an alligator
maxilla showing similarity of tooth and socket formation to A and B.