Intricacies in recording enamel hypoplasia in sheep molars.

Occurrence of enamel hypoplasia has been used as a marker of systemic stress during the period of tooth crown formation in various mammalian taxa, especially primates. In some studies, the macroscopically recorded position of hypoplastic enamel defects along the vertical tooth axis has been utilized to reconstruct the timing of stress events. Recently, this approach has been extended to bovid species with high-crowned cheek teeth. Enamel extension rates in these teeth are, however, unknown. This contribution presents results of light and scanning electron microscope studies on enamel hypoplasia in molar teeth of sheep from various locations and illustrates some aspects that must be considered when recording enamel hypoplasia in the teeth. Our findings indicate a significant variation in enamel extension rate, which is markedly reduced in cervical enamel. Therefore, occurrence of enamel hypoplasia is disproportionately high in this area. A general problem was the partial or complete filling of hypoplastic enamel defects with hyperplastic cementum and/or dental calculus, which hampered the identification of these defects on external inspection and precluded assessment of their depth. Only on sections it was possible to identify the accentuated incremental line (Wilson band) at the base of the hypoplastic defect, which marks the position of the enamel growth front at the time of the insult, and thereby to reconstruct the timing of the stress event causing enamel hypoplasia. Reliable assessment of the timing of stress events affecting tooth crown formation therefore requires histological analysis of tooth sections. [ABSTRACT FROM AUTHOR]