Biology Reference
In-Depth Information
more reliable count results with 7
m slices
(
Charles et al., 1986; Condon et al., 1986
). Despite these findings, many researchers opt
instead for nondemineralized slices with a thickness between 50 and 100
m slices than with nondemineralized 80
m
m
m(
Wittwer-Back-
ofen et al., 2004; Maat et al., 2006; Meinl et al., 2008
). This is likely due at least in part to the
increased time and cost involved in preparing the demineralized slices.
One further consideration is that similar to other aging techniques available for the skel-
eton, the accuracy of age estimations based on cementum annulation decreases as chronolog-
ical age increases (
Stein and Corcoran, 1994
). Obertov
ยด
and Francken (2009) tested this
method on a sample of 116 teeth from 65 individuals. The method worked relatively well
for individuals under the age of 40, but underestimated the ages of those individuals over
40 years. Again, as the processes governing the development of the annulations are poorly
understood, it is hard to determine exactly what factors contribute to this age-related decline
in accuracy. Does it have to do with diet? Perhaps age-related loss of other teeth and subse-
quent changes in jaw architecture and biomechanics? Both of these research questions need
to be addressed. Understanding the exact nature of the alternating dark and light bands,
the periodicity of the bands, and the age-related change in their expression will enable the
use of cementum annulations with greater confidence. This is an interesting area of research
with many questions awaiting answers.
m
Computed Tomography
The high-resolution images obtainable by computed tomography (CT) provide promise
for the study of dental tissues. There is a discrepancy between appearance of calcification
in X-rays and CT scans, making it potentially difficult to reconcile the population standards
of extant primates obtained via radiographic analysis with the stunning CT images of fossil-
ized, mummified, or otherwise preserved remains. All the same, both CT and microtomog-
raphy (
CT) present researchers with exciting ways to analyze dental growth and tooth form.
Conroy and Vannier (1987)
performed a series of scans on the Taung child (a juvenile Aus-
tralopithecus africanus discovered in South Africa, dated to ~ 2.5 million years ago) in order to
more clearly visualize the developing dentition and sinuses and more accurately characterize
its development. This imaging technique enabled researchers to distinguish between the
teeth and embedded matrix to show that the dental development evidenced in this specimen
was not nearly as human-like has had been previously put forth by others (
Dart, 1925;
Conroy and Vannier, 1987
).
Less than a year later, Conroy used CT analysis to examine the developing dentition of
another hominin specimen, SK61, a juvenile Paranthropus robustus, whose dentition had
been central to the discussion of whether robust australopithecines presented with a more
human or more ape-like developmental pace (
Conroy, 1988
). Because the developing teeth
still embedded in the jaws were not clearly visible in previous radiographic analyses,
previous assessments of the specimen had either incorrectly identified the incisors in occlu-
sion as the permanent incisors, or underestimated the stage of development of the I1 (
Broom
and Robinson, 1951; Dean, 1985
).
This created an interesting situation that would indicate that P. robustus had a more
human-like developmental schedule involving more advanced incisor development for the
M1 stage relative to the condition in both apes and gracile australopithecines. In humans,
m
