Tooth enamel biomineralization in extant horses: implications for isotopic microsampling

Abstract

Isotopic analyses of tooth enamel from fossil equids are increasingly being used to reconstruct paleoclimatic and paleoenvironmental conditions. However, the accuracy of these reconstructions is currently limited, partly because the precise timing and spatial patterns of enamel mineralization in equids have not been documented. We used radiographic analyses of mandibles collected from modern juvenile and adult domestic horses ( Equus caballus) to document the timing of enamel mineralization in equid cheek teeth (premolars and molars). Optical and radiographic analyses of thin sections of mature and developing teeth were used to document the spatial pattern of enamel mineralization in each tooth. We found that the enamel layers in equine cheek teeth took longer to mineralize than had been previously assumed, largely because the enamel continued to mineralize for approximately 6 to 12 months after each tooth had begun to erupt. Total enamel mineralization times for individual teeth ranged from ∼1.5 to ∼2.8 years. Examination of thin sections revealed that Retzius' striae extend outward from the enamel–dentin junction at an angle of ∼5° to ∼10° and run near-parallel to the nonocclusal surface of the tooth. Daily cross-striations average ∼5 μm in width, suggesting that, during the initial phase of enamel mineralization (matrix formation), new material is added at a rate of ∼5 μm/day. Radiographic analyses demonstrate that the secondary mineralization front (enamel maturation front) is orientated approximately parallel to the Retzius' striae, but complete maturation (i.e., full mineralization) lags behind matrix formation by several weeks to months.