Reconstructing infant mortality in Iberian Iron Age populations from tooth histology

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The Neonatal Line (NNL) of the tooth serves as a unique identifier, allowing us to distinguish whether a child survived birth. This line is essential for assessing the age at death of infants from skeletal remains found in archaeological contexts. Our primary objective is to accurately determine the age of infant intramural inhumations from the Iberian Iron Age (8th-1st centuries BC) by analyzing histological sections of tooth germs. Due to their fragility and high susceptibility to taphonomic factors, these samples are challenging to handle. By accurately assessing their age, we aim to classify individuals into various stages of infant mortality, which will help reconstruct infant mortality patterns in these populations.We analyze unerupted and still-forming crown deciduous teeth from 45 infant burials. By calculating Crown Formation Time (CFT) and identifying the NNL, we determine both gestational and chronological ages. We further validate the reliability of NNL identification through Synchrotron X-ray Fluorescence (SXRF) elemental analysis (Ca, Zn, Cu) on two contemporary and two archaeological samples.Our histological study reveals the chronological age of 38 infants from Iberian settlements, ranging from the 30th week of gestation to the 2nd postnatal month. The age distribution shows an attritional mortality pattern, with nearly half experiencing perinatal mortality, including preterm births. These findings support the hypothesis that mortality was primarily attributed to natural causes. Our research enhances the understanding of infant life history events in prehistory by combining histological analysis of tooth NNL and CFT, highlighting the technique's potential and limitations.