The age-at-death was estimated using two sets of data:

1) Post-natal cuspal enamel formation time in the paracone of M1: Post-natal cuspal enamel formation time is equal to the cuspal enamel formation time minus prenatal enamel formation time. We first measured the cuspal enamel thickness of the M1 paracone and then used the equation described in the previous section to obtain its formation time (see the previous section for more details of the equation). The cuspal enamel formation time of the M1 paracone is 315 ± 7 days (Table 1). Then, we identified the neonatal line, which can be observed on both enamel and dentine in the M1 paracone (fig. S1). The distance between the enamel neonatal line and dentine horn tip is measured as 48 μm, and therefore, the prenatal enamel formation time for M1 is 18 days based on the equation presented above. Subtracting 18 days of prenatal enamel formation time from two different estimates of cuspal enamel formation time (315 ± 7 days) from the two sets of authors yields a postnatal cuspal enamel formation time estimate for the M1 paracone of 297 ± 7 days.

2) Total lateral enamel formation time: The root surfaces of Xujiayao permanent teeth are poorly preserved, making the counting of periradicular bands imprecise, so it is impossible to obtain the age-at-death based on one tooth (e.g., M1). Therefore, we cross-matched the first-formed M1 paracone (buccal surface) until the last-formed M2 hypocone (distolingual surface) perikymata through the buccal surfaces of C and P4 using accentuated perikymata and LEH (fig. S2 and data file S1). The virtual reconstructions were used in the cross-matching, and they were obtained from synchrotron scanning at a resolution of 6.34 μm and μCT scanning at a resolution of 43.0 μm (data file S1). Because the crown of the M2 hypocone had not yet completed its formation, only counts of perikymata were involved in calculating age-at-death (fig. S2). Note that Fig. 2 shows the development of the M2 paracone (mesiobuccal cusp), not the hypocone (the distobuccal cusp). The total lateral enamel formation time across all teeth is 2080 ± 40 days, calculated as follows: (10 days × number of perikymata counted on the M1 lateral enamel to LEH1) + (10 days × number of perikymata counted on C′ between LEH1 and LEH4) + (10 days × number of perikymata counted on P4 between LEH4 and LEH6) + (10 days × number of perikymata counted on the M2 hypocone between LEH6 and the end of enamel development). The error in total lateral enamel formation time derives from differences between the two sets of observers in their counts of perikymata.

The two sets of data postnatal cuspal enamel formation time and total lateral enamel formation time were added together to obtain the age-at-death: 2377 ± 47 days or 6.51 ± 0.13 years.

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