Biology Reference
In-Depth Information
from different populations is crucial to expanding our knowledge and accuracy of interpre-
tation of sequence polymorphisms. In addition to the benefits that this information would
provide to the study of modern human populations, it would give us greater insight into
the differences observed in fossil hominins.
VISUALIZING TOOTH GROWTH
Radiographic Methods
The study of radiographs is the most common method used for the development of pop-
ulation standards of dental growth. Images are relatively inexpensive and less time
consuming to obtain in comparison with computed tomographic (CT) images, and histolog-
ical and gross methods. It follows that inclusion of a higher number of individuals is usually
possible. The cost of high-resolution digital radiographs is lower than that of CT scans and
a number of truly portable machines are available commercially.
In addition to portable radiographs, there are table-style veterinary X-ray units with
stationary beams (which are actually quite large and cumbersome when considering a project
that involves collecting images at several sites). Additionally, handheld dental X-ray systems
are available that are small enough to fit into a case the size of a carry-on style suitcase. The
handheld models are perfectly suited for one who is interested in obtaining dental X-rays on
skeletal specimens. This method is nondestructive, thus making it suitable for use in studies
of museum collections, delicate specimens, and fossils. However, embedded matrix or sedi-
ment in fossil or archaeological specimens can sometimes prevent full visualization of the
desired features. In these cases, other methods that can better distinguish between these
materials must be sought out as discussed below (
Conroy and Vannier, 1987
).
One drawback is that radiographic analysis consistently underestimates the degree of
calcification of teeth when compared with gross or histological methods (
Simpson and
Kunos, 1998
). Calcification is the deposition of hydroxyapatite mineral that makes up a large
portion of enamel and dentin. We look at the degree of calcification to determine the state of
development of the root and crown. However, it is still a practical method because it allows
for a relatively inexpensive and less time-consuming way to obtain greater amounts of data
than would be possible through other analytical methods. Furthermore, available population
standards are almost entirely based on the radiographic appearance of calcification and they
present a clear and functional account of the calcification that can be useful in intra- and inter-
population comparisons. As such, it remains the standard for producing population stan-
dards of dental development.
Analysis of Dental Development
Generally speaking, two different methods exist for the analysis of dental development via
X-ray imaging: the atlas approach and the dental age approach. First, the atlas approach
utilizes a set of developmental markers with associated ages as a basis for comparison
with the individual. The individual is assigned a developmental age depending on where
it fits most appropriately based on the markers in the predetermined model. Perhaps the
most widely recognized atlas of human dental development is that of
Schour and Massler
