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Dental age determination
This protocol is extracted from research article:
Calcium isotopic patterns in enamel reflect different nursing behaviors among South African early hominins
Sci Adv, Aug 28, 2019;

Procedure

Before and after sampling, a picture was taken for each tooth containing, when possible, the two microsampling holes. All the pictures were taken with the same magnification. The pictures were imported in Adobe Illustrator software. Several points were located on the pictures: (i) the cusp tip of the tooth crown (point A; fig. S8); (ii) the lowest point on the buccal or lingual enamel (point B; fig. S8); and (iii) the positions of the microsampling holes. The microsampling hole close to the cusp tip is annotated “upper” (point U) and that close to the cervix is annotated “lower” (point L; Tables S1 and S2 and fig. S8). Then, a line was drawn from A to B, following the long growth axis of the enamel. The positions of the points U and L microsampling holes were projected to that line, parallel to external incremental growth marks, when visible. The projected U and L positions on the AB line are marked U′ and L′. Last, the distances from A to B (tooth height, t), from the points B to L′ (a), and from the points B to U′ (b) were measured in arbitrary units and reported on the picture. A complete list of the pictures showing these measurements is given in the Supplementary Data. The resulting relative positions (i.e., height of the projection of a microsampling hole relative to the tooth height, H%) were replaced in the chronology of dental development of early Homo (22, 23, 26, 28), A. africanus (26, 27), and P. robustus (26, 27). Dental development determination can be based on two different approaches: The first is based on synchrotron virtual imaging using counts of long-period (Retzius lines) and short-period (cross striations) internal incremental lines (26), and the second is based on counts of perikymata (22, 23, 27, 28). Using the charts provided in these references for a given taxon [early Homo: Fig. 10.9 (22), Fig. 4 (23), Table G (26), Table 2 (26), and Tables 1 and 2 (28); A. africanus and P. robustus: Fig. 4 (27), Table G (26), and Table 2 (26)], the ages of dental development can be reconstructed independently. Both approaches yield similar results of crown initiation age and crown formation time for incisor, canine, premolar, first, and second molar (fig. S9).

Important uncertainties are associated with reconstructed dental ages for three main reasons. First, most of the teeth are worn, which precludes an accurate estimation of the original crown height. Second, for a given tooth type, the chronological age cusp tip must incorporate the time taken to form cuspal enamel to the initiation age. Third, the sampling spots were performed at a more or less constant depth, but this was not formally controlled and could lead to the sampling of varying amounts of enamel layers, notably of cuspal enamel. To integrate all these uncertainties and the variations of the two calendars of dental age, we therefore consider for each spot the youngest crown initiation age and the longest crown formation time specific of the tooth type. The minimum dental age (min) and maximum dental age (max) for each tooth are summarized in table S2. The relative positions of the microsampling holes were converted into dental age using the following equation$Dental age=%H*[max−min100]+min$

The complete list of dental age calculations is given in table S3.

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