The Xujiayao 1 maxilla was scanned by μCT for assessing developmental stages of all deciduous and permanent teeth (Fig. 1 and data file S1). All six permanent teeth were scanned with PPC-SRμCT on beamline ID19 of the European Synchrotron Radiation Facility (Grenoble, France) using a voxel size of 6.34 μm (data file S1). A 0.742-μm voxel size of PPC-SRμCT scanning was performed on the cuspal apex of the M1 paracone to image the neonatal line (fig. S1), on the cuspal apex of the M2 paracone for the enamel daily secretion rate (DSR), and on the lateral enamel of M2 for long-period line periodicity (fig. S1 and data file S2). Thirty-micrometer-thick slices were generated in VGStudio 2.2 and VGStudio MAX 3.0 for the observation of periodicity and neonatal line. Two hundred–micrometer–thick slices were used in the measurement of enamel thickness and root length.

Regarding the relatively recent nature of the Xujiayao fossils, there may be a possibility of still having ancient DNA (aDNA) traces preserved. In order not to endanger any future aDNA analysis on this specimen, special care during the synchrotron experiment was taken to ensure that low-dose delivery was possible while still reaching sufficient data quality to perform the dental development analysis, following the recommendation of Immel et al. (35) and using the same dose quantification protocol and material.

Considering that the detection limit for effect of x-rays on aDNA is 200 Gy (water equivalent surface dose), we adapted the synchrotron scanning protocols for full teeth imaging to stay below this limit (about 150 Gy), keeping in mind that, up to 2000 Gy, the damage is negligible and would not endanger aDNA study. The dose given to the whole specimen during the conventional scan was not measured but can be estimated from the scanning parameters to be 1.4 Gy, i.e., without any detectable effect on aDNA (data file S1). Dose being cumulative, the complete scan history of the specimen has to be taken into account and recorded. Only the scans performed at submicrometer resolution resulted in accumulated dose above the detection limit, but on very restricted volumes each time (about 3.5 mm × 1.6 mm), and located mostly in enamel. In these areas, the total accumulated surface dose reached a maximum of about 5500 Gy (probably less due to beam attenuation by the sample itself), corresponding to a maximum aDNA loss of 20% [see (35) for more details]. Orientation of the teeth for submicrometer resolution scanning was optimized to reduce as much as possible the amount of material crossed by the beam during the scans, to ensure that only the scanned area would receive an x-ray dose, potentially leading to some aDNA degradation.

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