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saurs (Figure 1E-G). This is not surprising when we consider
that it is merely an elaboration of bone that was already involved
with fixing the teeth to the jaw. In camosaurs, this expansion
forms structures that have been variously termed "interdental
rugosae" (Osborn, 1912), "interdental plates''
(Madsen, 1976), or "infradental plates" (Gardiner, 1982). In
the mandible, these "plates" lie on top of the dentary and are
slightly labially inset to the lingual wall. In the upper jaw, they
lie beneath the maxilla and premaxilla, slightly labial to the lingual
walls (Figure lF,G). They are generally bounded anteriorly
and posteriorly by vertical grooves leading into foramina at the
base of the plates. The foramina also are connected by a horizontal
groove on the ledge at the base of the interdental plates.
Each foramen is paired to one tooth site and commonly contains
a developing tooth (Figure lG). The grooves and foramina
may mark the sites for the dental lamellae, an interpretation
that is consistent with their termination at the location of newly
deposited tooth crowns. Because the grooves are at the tooth
sites of the jaw, the flat attachment bone between them is "interdental."
Interdental plates of this sort occur in most saurischians
and in many thecodonts (Martin et al., 1980). The only
significant variation we have seen in the morphology of interdental
plates is the occasional obliteration of the vertical
grooves in presumably older individuals. That the interdental
plates are continuous with the interdental septae and distinct
from the tooth-bearing bones themselves was observed by Osborn
(1912) and Walker (1964).
Each method of lingual wall formation is accompanied by a
characteristic mode of tooth replacement. In fact, in the crocodilian
mode of replacement, the new tooth has most of its formation
in the pulp cavity of its predecessor. This mode of replacement
also is facilitated by the expanded root and was
described by Edmund (1960:114-115) thus: "The crown of a
replacement tooth develops within the body of the old tooth,
mainly below the neck separating the wider base from the narrower
crown. In this way the diameter of the replacement
crown can become greater than that of the crown of the tooth
within which it lies." The signature feature of this type of replacement
is a pit that completely surrounds the developing replacement
tooth (Figure 2E,F), a feature that is absent in all of
the many thousands of known dinosaur teeth. Edmund
(1969:186) pointed out that saurischian dinosaurs differ from
crocodilians in that the replacement tooth did not enter its predecessor's
pulp cavity at an early stage, but seems to have been
associated with progressive lingual resorption, with the resulting
appearance of having dissolved its way into the lingual
wall. The new tooth does not become central in the alveolus
until it is about half grown, and much of its predecessor has
been resorbed. Frequently a replacement tooth can be seen in
the alveolus lingual to its predecessor, the latter being still perfectly
functional. From the discussions of Edmund (1960,
1969), and from examination of many saurischian specimens, it
is clear that the replacement teeth of saurischians form and
continue in an upright position to their maturity. In camosaurs
FIGURE 2.—A-D, Teeth of theropod dinosaurs thought by various authors to be
especially close to birds: A, Mononykus (modified from Perle et al., 1993); B,
Troodon; C, Saurornitholestes; D, Dromaeosaurus (C-D modified from Currie
et al., 1990). Teeth showing constricted crown, replacement tooth tip, and
expanded base: E, bird, Parahesperornis alexi Martin; F,G, crocodilian, Alligator;
G, lateral cross section showing the tilted replacement tooth resorbing the
root of its predecessor (modified from Edmund, 1962).
the replacement teeth form rows on the lingual side of the mature
tooth, and we have seen as many as three generations of
teeth ranked side by side. In crocodilians, however, the replacement
tooth prepares to enter the pulp chamber of its predecessor
by first tilting toward it (Figure 2G). The developing crown
then passes in and upward through a circular resorption window
in its predecessor (Figure 2F). The teeth of crocodilians
are attached by periodontal ligaments running from the jaw
bones to the root cementum on the expanded roots (Miller,
1968). This mode of attachment has not been recognized in
other diapsid reptiles, which also may lack the necessary root
cementum.