This group of values was based on the cross-matching of LEH and accentuated perikymata on all of the Xujiayao permanent teeth (fig. S2 and data file S1). The time spent for all teeth between birth and the same LEH (e.g., LEH1), composed of initiation time, cuspal enamel formation time, and lateral enamel formation time before the LEH is the same. The initiation time of M1 is −18 days (see the previous section for more details). Using the initiation of M1 and the cross-matching of the LEH across the other teeth, the initiation time of all the other Xujiayao permanent teeth can be calculated. For example, by cross-matching I1 against M1 (having neonatal line) using LEH1, I1’s initiation time can be calculated to be 283 days = 308 days − 18 days (cuspal formation time of the M1 paracone after birth) + 68 perikymata (first perikymata to LEH1 of the M1 paracone) × 10 days − 377 days (cuspal enamel formation time of I1) + 31 perikymata (first perikymata to LEH1 of I1) × 10 days. In fig. S2, we did not provide the buccal view of the M2 paracone to provide a whole profile of cross-matching using the estimation of age-at-death. However, we provided the perikymata number (77 lines) between the paracone apex and LEH6. By cross-matching P4 and M2 using LEH6, the initiation time of M2 is calculated to be 920 days = 829 days (initiation time of P4) + 371 days (cuspal enamel formation time of P4) + 85 (first perikymata to LEH6 of P4) × 10 days − 360 days (cuspal enamel formation time of the M2 paracone) + 77 (first perikymata to LEH6 of the M2 paracone) × 10 days. The interobserver error varies from 1 to 54 days (Table 1 and table S1), depending on the difference on measurement of cuspal enamel thickness and perikymata counting.

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