Abstract
Quantification of dental long-period growth lines (Retzius lines in enamel and Andresen lines in dentine) and matching of stress patterns (internal accentuated lines and hypoplasias) are used in determining crown formation time and age at death in juvenile fossil hominins. They yield the chronology employed for inferences of life history. Synchrotron virtual histology has been demonstrated as a non-destructive alternative to conventional invasive approaches. Nevertheless, fossil teeth are sometimes poorly preserved or physically inaccessible, preventing observation of the external expression of incremental lines (perikymata and periradicular bands). Here we present a new approach combining synchrotron virtual histology and high quality three-dimensional rendering of dental surfaces and internal interfaces. We illustrate this approach with seventeen permanent fossil hominin teeth. The outer enamel surface and enamel-dentine junction (EDJ) were segmented by capturing the phase contrast fringes at the structural interfaces. Three-dimensional models were rendered with Phong’s algorithm, and a combination of directional colored lights to enhance surface topography and the pattern of subtle variations in tissue density. The process reveals perikymata and linear enamel hypoplasias on the entire crown surface, including unerupted teeth. Using this method, highly detailed stress patterns at the EDJ allow precise matching of teeth within an individual’s dentition when virtual histology is not sufficient. We highlight that taphonomical altered enamel can in particular cases yield artificial subdivisions of perikymata when imaged using X-ray microtomography with insufficient resolution. This may complicate assessments of developmental time, although this can be circumvented by a careful analysis of external and internal structures in parallel. We further present new crown formation times for two unerupted canines from South African Australopiths, which were found to form over a rather surprisingly long time (> 4.5 years). This approach provides tools for maximizing the recovery of developmental information in teeth, especially in the most difficult cases.
Highlights
Scholars have long recognized the wealth of information preserved in dental hard tissues
Since high resolution data for cuspal enamel were not available, we based these measures on cuspal Retzius lines, observed on two orthogonal cut planes (S11 Fig and S1 Supporting Information, section III). This yielded an average Cuspal daily secretion rate (CuDSR) of 3.57 μm/d obtained on the maxillary canines of STW151, MLD11-30 and STS2. Since this approach remains less precise than direct observations of cuspal cross-striations along prisms, we investigated the effect of using different CuDSR found in the literature to provide a range of crown formation times (CFT) that maximizes the probability of containing the true value
We report a wider range of CFT taking into account CuDSR of Pan and recent human canines as: 4.86–5.16 (±0.25) years for the MLD2 LLC and 4.61–4.92 (±0.15) years for the StW151 LLC
Summary
Scholars have long recognized the wealth of information preserved in dental hard tissues. In order to determine growth periods and/or age at death of juveniles using a direct measurement independent of modern human or great ape standards, long-period incremental growth lines that course through the enamel (Retzius lines) and manifest on crown surfaces (perikymata) are the most commonly counted developmental features. Their periodicity needs to be determined for calculating the crown formation time. This is achieved by counting the daily prism cross-striations between two successive long-period lines. These techniques are destructive, and are only rarely applied to valuable fossil specimens
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