Exploring enamel inside and out: evidence of developmental disruption among archaeological barren-ground caribou (Rangifer tarandus groenlandicus) from southern Baffin Island, NU

Abstract

Teeth are commonly excavated items among archaeological assemblages and provide key insights to the individual from which they develop. Specifically, dental enamel registers evidence of biorhythmic growth (and disruption) within the tooth crown morphology as enamel is deposited. The present research investigates evidence of systemic growth disruption among the molars of archaeological barren-ground caribou (Rangifer tarandus groenlandicus) excavated from the LdFa-1 site in southern Baffin Island, Nunavut, Canada. This study is the first to examine Rangifer crowns from the inside-out, exploring both the internal composition of enamel and surface morphology. With minimal prior data available, the overall focus of this project is to begin establishing locational patterns and frequencies of developmental disruption observed among each molar type (M1-3). Examining growth disruption at the enamel surface is accomplished by generating a profile of each tooth using laser-scanning confocal microscopy (LSCM) and assessed with two recently developed overlay techniques (a 6th order polynomial trendline and a spline curve) (Cares Henriquez and Oxenham 2020; Gamble and Milne 2018). Histological thin sections are then created and observed under polarized light to identify evidence of developmental disruption among the internal enamel matrices. A disruption event is represented by a dark line (accentuated stria of Retzius) where enamel secretion experienced abnormal pause. This study finds the spline curve to be a better fit for analyzing surface profiles via LSCM compared to 6th order polynomial, but the technique produces inaccurate evidence of enamel growth disruption. However, of 39 accentuated striae of Retzius terminating at the outer enamel surface, 23 could be associated with surface defects. This suggests that an LSCM spline curve may be a useful analytical tool for future research, but should only be applied as a supplemental approach to a more established method of identifying enamel growth defects (such as scoring accentuated striae of Retzius). This study incorporates such an approach to the present data, producing initial locational frequencies of surficial developmental disruption (referred to as linear enamel hypoplasia, or LEH) among Rangifer teeth. These trends are then interpreted based on general, yet highly synchronic caribou life-history patterns with potential to result in growth disruption.