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296 SMITHSONIAN CONTRIBUTIONS TO PALEOBIOLOGY
FIGURE 1.—A-E, Cross-sectional diagrams showing relationships of the
tooth-bearing bone (unshaded), attachment bone (black), adult tooth (diagonal
hatching), and replacement teeth (stippled): A, stem-reptiles (acrodont); B, earliest
diapsid reptile (Petrolacosaurus); c, crocodilians and birds (thecodont
with a groove); D, lizards (pleurodont); and E, "thecodonts" and dinosaurs (thecodont
through superpleurodonty). F, cross section of a dentary of Troodon
showing the replacement tooth standing lingual to its predecessor (modified
from Currie, 1987, fig. Id). G, medial view of the maxilla of the theropod dinosaur
Megalosaurus (modified from Charig, 1979:15), showing replacement
teeth (rt), interdental groove (ig), and interdental plate (ip).
sion of the labial margin of the tooth-bearing bones so that the
teeth rest on a shelf (Figure IB). Replacement teeth form lingually
to the teeth they replace, and this probably inhibits the
formation of an inner bony wall. The teeth are attached to the
bony shelf by a highly cancellous bone called "bone of attachment"
(Tomes, 1923). Attachment bone also surrounds the developing
teeth, which are able to migrate labially through it.
Edmund (1969:126) briefly discussed attachment bone, saying
that it "appears to be embryologically and histologically identical
with the alveolar bone of mammals and thecodont reptiles.
It develops from the dentigenous bone near the base of the new
tooth, fuses with it, and is, in turn, resorbed in the process of
shedding of the old tooth." It also attaches the roots of the teeth
of lizards to the labial side of the jaw, thus forming the pleurodont
condition (Carlson, 1990). In mammals the periodontal
sac encloses the developing tooth and deposits bone (cementum)
around the root using the mesenchymal tissues on its inner
surface. Imbedded in the cementum are collagenous fibers
from the periodontal sac, called periodontal ligaments, that also
become imbedded in the bone laid down by the external surface
of the sac (Carlson, 1990), which corresponds to bone of
attachment. Mammalian teeth have a very limited replacement
sequence and have dense, well-formed alveolar bone. Attachment
bone of animals with continuous replacement is constantly
being remodeled. It has the distinct porous (fibrous) morphology
of bone that is subject to rapid resorption and
regrowth.
The tooth when embedded in the jaw is within the dental sac
and thus can be surrounded by attachment bone. The dental
lamellae form tooth papillae on the lingual side in an ordered
sequence. This is generally tme for all vertebrates, and primitively
much of the formation of a replacement tooth occurs lingual
to the tooth it will replace. The tooth migrates through the
easily reconfigured attachment bone and is always internal to
the dense bone of the jaw.
Because the tooth family is lingual, it is not obstmcted by a
labial wall supporting the root. An outer wall helps the teeth to
resist outward strain, but stabilizing the teeth to inward pressure
is attained either by fusing the teeth (Figure ID) to the labial
wall (pleurodonty) or by building an inner wall lingual to
the tooth row (thecodonty). In the latter case, space for the
tooth family must still be provided.
Archosaurs evolved two solutions to the problem of building
an inner wall. One method was to bring up the inner edge of
each tooth-bearing bone to form a groove (Figure lC). In its
most primitive stages this groove may not have contained complete
septa, although it seems likely that the groove would have
had some constrictions around the teeth. This is essentially the
situation that we see in young crocodilians and in young birds.
In these archosaurs, the required anteroposterior stabilization
of the dentition is provided in part by expanding the roots of
the teeth so that they nearly contact one another, and this also
gives more surface for periodontal ligaments. The more advanced
condition is seen in adult crocodilians. Here, lingual
and labial projections meet to form septa, and teeth of adult
crocodilians tend to have less bulbous roots than do the teeth in
juveniles (Martin et al., 1980). An alternate solution is expansion
of the attachment bone until it forms the lingual wall,
which is found in several archosaur groups, including dino-