TAXON
Infant
Juvenile 1
Juvenile 2
Adult
Papio 0 -2.0 2.01 -4 .5 4.51 - 7 .0 +7.0
Pan 0 - 3 . 5 3.51 -7.0 7.01 - 1 0 . 5 +10.5
Table 2.2
Age groups based on dental development for Papio & Pan.Defining these ARDAs placed all mandibular growth and permanent molar mineralization data into an identical time scale. Thus, rates of mandibular growth could be determined for dimensions in regions close to the site of permanent molar development. Scatterplots of rates of mandibular growth demonstrate how different rates (and ultimately different sizes) among the three taxa contribute to differences in adult morphology. These data were compared with mesiodistal molar crown lengths. Then both absolute and relative mandible length:molar row proportions were compared among all three taxa in individuals that showed either M1, M2 or M3 initiation. This is presented in Chapter 4 and was the first of two analyses conducted using the ARDAs. This first analysis included a bivariate study of absolute rates of mandibular growth and times of permanent molar crown initiation in Papio and Pan. Individual ages were plotted against mandibular measurements using Microsoft Excel 97. The selection of individuals and measurements included in this study is elaborated upon later in this chapter. The measurement of the mandibles and molar teeth of all the specimens included in this study are described in detail in the following section.
The second analysis was a three dimensional (3D) multivariate study of mean mandibular shape change over time in Papio and Pan. This study compared shape change across four mean mandible shapes, each derived from and representative of the above four age groups. This emphasised ontogenetic shape changes in the jaws of Papio and Pan, thus clarifying visual as well as statistical analyses of mandible shape. The multivariate 3D statistical shape analyses included in this thesis research are discussed in detail after the description of the linear measurement of the mandibles and molar teeth.
2.2.2 Linear measurement of the mandibles and molar teeth
A total of twenty-five linear measurements were taken from the mandibles (sixteen measurements) and permanent molar teeth (nine measurements) of the majority of specimens that had been digitized and radiographed. These measurements are described in detail in Table 2.3 and illustrated in Figures 2.2 and 2.3. All measurements were taken on the right side of each specimen unless the right mandible was damaged, in which case measurements were made on the left side of the jaw. The linear measurements, taken with manual calipers and an osteometric board, gave individuals absolute scales. These measurements were supplementary to the 3D shape data. Perhaps most importantly, these measurements enabled the independent study of both relative and absolute changes in mandibular and molar row proportions over time, and were used to test some of the conclusions drawn from the statistical shape analyses.
Measurement data were analysed using Microsoft Excel 97. This software was used to calculate descriptive statistics from the measurement data and to plot measurements against one another and against ARDAs. Time of crown initiation was most important to this study. Either sufficient space for a permanent crown must be available in the mandible before molar initiation begins or rates of mandibular and molar crown development must be such that space is created in the jaw slightly in advance of, or simultaneously with, crown formation. At some time during or very near the interval between M2 crown initiation and M2 crown completion, M3 crown development begins. Thus, changes in mandibular proportion during this period of dental development were most salient to this study. Means were calculated for measurements of posterior corpus length (POSTL) and mesiodistal length of the emerged molar row (MDM). Mean ratios of MDM to POSTL were calculated at stages of ‘M1 in occlusion', ‘M2 in occlusion’ and ‘M3 in occlusion’. Each of these stages corresponded directly to MDM, the mesiodistal length of any and all emerged permanent molar teeth.
To test measurement error, eight individuals were each remeasured three times on different days, in different orders. All specimens were Papio
anubis, and included a broad range of ages. All sixteen mandibular measurements were included for each individual. However, because of variation in dental maturity, and because not all molar teeth were emerged in all individuals, some molar dimensions could not be measured for some individuals. Standard deviations (SDs) for each measurement were calculated for each individual. Mean standard deviations were then calculated for each measurement (White & Folkens 2000). The greatest mean standard deviation was 0.06 cm for total mandible length (MANDL). This was not unexpected as MANDL was the greatest dimension measured, and for this reason should be prone to the greatest measurement variance. This relatively large mean SD was probably also due to measurement technique; MANDL was the only measurement taken with an osteometric board, an instrument that is less precise and much more unwieldy than are calipers. All other measurements varied around 0.04 cm or less, many of these around 0.01 cm and 0.02 cm. Thus, measurement error should not significantly affect results. Appendix 3 includes the raw measurement data, and its means and standard deviations. ‘Variance’ describes the variance about the mean of all eight sets (one set per individual) of the three repeated measures for each mandibular measurement (i.e. NOTD). ‘Mean SD’ describes the mean of all (eight) standard deviations (SD) for each mandibular measurement. ‘VARA for SD’ describes the variance among the eight standard deviations derived from the eight sets of repeated measures for each mandibular measurement. This statistic is negligibly small for most if not all mandibular measurements.
Table 2.3 List of mandibular and permanent molar crown measurements.
MEASUREMENT
NUMBER
MEASUREMENT NAME
(ABBREVIATION)
MEASUREMENT DESCRIPTION
1 34
Notch depth (NOTD)
Body width (BOOTH)
Condyle width (CONW) Condyle-coronoid length
(CNCR)
Body height (BOOH)
Bimandibular breadth (BIM1)
The maximum depth of the mandibular notch from a line joining the superor- most points on the mandibular condyle and the coronoid process to the inferior-most point of the mandibular notch.
The minimum thickness of the mandibular corpus measured across M1 or the gubernaculum of the same tooth with the jaws of the calipers orthogonal to the occlusal plane.
The maximum mediolateral width of the condyle.
The distance from the superior-most point on the condyle to the superior- most point on the coronoid process.
The height of the corpus from the inferior-most point of the crest of the buccal alveolar bone opposite the mesiobuccal root of M1, o r the inferior border of the gubernaculum of M1 to the lower border of the mandible.
The bimandibular breadth between the left and right bodies of the mandible from the inferior-most point on the crests of the alveolar bone opposite the mesiolingual root, or the equivalent end of the gubernaculum, of M1.
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Symphysis height (SYMHT)
10
11
12
13
Rama! width (RAMAP) Notch height (NTHT)
Bilingual breadth (BILING)
Condyle height (CONHT)
Bicoronoid breadth (BICOR)
Mandible length (MANDL)
The perpendicular height of the mandibular symphysis from the alveolar crest between the (deciduous or permanent) central incisors and the inferior-most point of the lower border of the symphysis in the midline, measured with the mandible resting on a level surface.
The minimum anteroposteror width of the ascending ramus.
The height of the ramus from the inferior-most point of the mandibular notch to the tubercle or most protruding part of the inferior border of the ramus (tubercle defined after Hrdlibka 1940).
The minimum distance between the base of the right and left lingulae at the opening of the mandibular foramina.
The height of the ramus from the superior-most point of the condyle to the tubercle or most protruding part of the inferior border of the ramus (after Hrdlidka 1940).
The maximum breadth of the mandible measured between the superior-most points of the right and left coronoid processes.
The minimum antero-posterior length of the mandible measured between a line perpendicular to the posterior-most points of the condyles to a line perpendicular to the anterior-most point of the mandibular symphysis (measured with a mandible board).
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14
15
16
17-19