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Implantation and Replacement of Bird Teeth
ABSTRACT
Study of the teeth of the Mesozoic birds Hesperornis, Parahesperornis,
Ichthyornis, Cathayornis, and Archaeopteryx provides
new evidence for avian tooth implantation and replacement. Birds
share with crocodilians and, to a lesser extent, mammals, a complex
mode of tooth implantation, with deep sockets walled lingually
by the dentary, maxilla, or premaxilla. These walls crowd
the replacing teeth so mat early in ontogeny the teeth migrate labially
and continue their development under the crown of their predecessor.
They thus form a vertical tooth family, as opposed to the
horizontal tooth family found in dinosaurs and most other tetrapods.
Birds, crocodilians, and mammals have root cementum on
their teeth and presumably attach teeth to the socket with periodontal
ligaments. The sockets in mammals and presumably in
birds are formed by the outside of the periodontal sac, whereas
cementum is deposited by the inside of the sac. Bird teeth are initially
formed in a groove, and ontogenetically the sockets (in
socket-forming species) form first at the front of the jaw. Socket
formation then proceeds posteriorly, as in crocodilians. Young
dinosaurs have the lingual side of the jaw around the teeth open, so
that the roots are exposed. The sockets form around dinosaur teeth
as bone of attachment, which is probably the same periodontal
bone that forms sockets in mammals, crocodilians, and birds. The
sites of new tooth formation extend lingually within the so-called
"special foramina" that separate the interdental plates. The interdental
plates represent the surrounding attachment bone and are
similar to the attachment bone in pleurodont lizards. In fact, dinosaurs
might be characterized as having a superpleurodonty that
results in sockets.
Introduction
In our previous paper on avian teeth (Martin et al., 1980), we
called attention to numerous features shared by crocodilians
and birds but not found in theropod dinosaurs. At that time, we
were unaware of how fundamentally different the whole dental
Larry D. Martin andJ.D. Stewart
Larry D. Martin, Natural History Museum and Department of Ecology
and Evolutionary Biology, University of Kansas, Lawrence, Kansas
66045, United States. J.D. Stewart, Los Angeles County Museum,
900 Exposition Boulevard, Los Angeles, California 90007, United
States.
295
system is in crocodilians and dinosaurs and how similar the
dentition is in crocodilians and birds.
The characteristic tooth morphology of crocodilians and
birds includes a flattened, unserrated crown that becomes constricted
as it approaches the crown/root juncture. The tooth is
narrow at this point and then expands into a cement-covered
root at least as broad as the crown and usually broader. Resorption
begins as circular to oval pits in the lingual side of the root,
and the replacement tooth has most of its formative history beneath
the tooth that it will replace (below or above depending
on lower or upper dentition). This morphology is found in all
of the Triassic and Early Jurassic crocodilians that we have
been able to examine. For instance, this tooth form is very
clearly shown in acid-prepared specimens from the Liassic
(Early Jurassic) marine crocodilian Pelagosaums in The Natural
History Museum, London, collections.
ACKNOWLEDGMENTS.—We are grateful to Alan Charig,
Cyril Walker, and Angela Milner of The Natural History Museum,
London (formerly the British Museum (Natural History);
BMNH) for access to specimens; P. Wellnhofer (Bayerische
Staatssamlung, Munich) generously shared access to specimens
and insights, as did P. Currie (Tyrrell Museum, Drumheller)
and G. Edmund (Royal Ontario Museum, Toronto). C.
Bennett and John Chom read the manuscript, and we especially
thank Zhonghe Zhou for helpful suggestions. The photography
staff at BMNH made the excellent ultraviolet photographs, and
the drawings are by Mary Tanner (Craneview Studio, North
Platte, Nebraska) and A. Aase (University of Kansas Natural
History Museum). Funding was provided by National Science
Foundation grant DEB 7821432 and National Geographic Society
grant 2228-80.
Discussion
Aside from the nature of the teeth themselves, their mode of
implantation in vertebrates also has proven to be useful in
working out relationships. The earliest reptiles had acrodont
teeth, as are found in the labyrinthodont amphibians and the
captorhinomorph reptiles (Figure lA). In the earliest diapsid
reptile known (Petrolacosaums), this condition has been modified
(Reisz, 1981) by the upward (in the lower dentition) exten-