Abstract:

The hominin fossils recovered from Xujiayao-Hsuchiayao (Locality 74093）site is critical in understanding the morphological variability of hominins from the period of Middle to late Pleistocene transition. Other than the morphologies, the pathological aspects of Xujiayao hominins were also investigated and the juvenile (Xujiayao 1) was believed to be suffered from dental fluorosis based on the presence of the yellow pit or furrow on its anterior teeth. The incidence of “dental fluorosis” in Xujiayao has been thought to represent the earliest evidence of this pathological anomaly. However, with the use of scanning electron microscopy (SEM) and microcomputed tomography (micro-CT), the yellow enamel defects were found to be hypoplastic alternation that occurred before the tooth eruption. They were not post-eruptive physical breakage resulted from chewing force and therefore don’t support the diagnosis of dental fluorosis. In addition, synchrotron phase-contrast microtomography of the anterior teeth under both micron and submicron resolutions didn’t show obvious sign of subsurface hypomineralization along the sagittal section of the enamel, and it doesn’t support the occurrence of dental fluorosis resulted from the disturbance of enamel maturation phase. However, plenty of pit-type hypoplastic defects were present on the enamel surface of the Xujiayao permanent teeth and the bottom of the enamel pits were underlain by accentuated incremental line. This type of enamel defects could be resulted from distributed secretory stage of enamel formation by excessive fluoride intake according to the experimental study on mammals. Apart from the surface defects, synchrotron scanning under submicron resolution at four different spots of Xujiayao permanent teeth reveals plenty of enamel holes inside the crown. These holes have a sphere-like shape and generally restricted to outer one-third area of the enamel thickness. Different locations within one scanning spot vary in the density of enamel holes. In canine, the enamel holes of high-density correspond with a hypoplastic depression on the enamel surface. Enamel holes inside the paracone apex of m¹ are in some cases connected with each other, with the main axis perpendicular to the outer enamel surface. These characteristics indicate a non-random distribution pattern of the enamel holes, and that they might be caused by the same effects as that of enamel hypoplasia. Teeth forming at different times vary in the density of hypoplastic pits and enamel holes, and this might imply various level of physiological disturbance at different stages of dental growth and development. Future study could further quantify the fluoride content in the deposit containing the Xujiayao hominin fossils, as well as in the tooth enamel, in order to ascertain if the Xujiayao people used to live a fluoride-rich environment and if they did ingest enough fluoride. With this information, the mechanism of enamel defects in Xujiayao juvenile could be more thoroughly understood.

Key words: Xujiayao hominin, Dental fluorosis, Synchrotron, Enamel defect