Variation in the fusion sequence of primary and secondary ossification centers in the human skeleton

OBJECTIVES: One of the biggest challenges for biological anthropologists is determining the minimum number of individuals in commingled skeletal samples in forensic or bioarcheological contexts. The fusion sequence of primary and secondary ossification centers is a promising, yet under-explored, process to improve identification of associated remains of subadults and young adults, but is limited by the lack of understanding of population variation in this aspect of human development. While prior studies show within-population variation in fusion sequence, possible geographic variation has not been systematically explored. MATERIALS AND METHODS: To explore potential variation in fusion sequence in different populations, we analyzed eight skeletal samples of East Asian, North American, African, and European ancestry. Forty-three epiphyses were cross tabulated to determine the order of beginning and completing fusion for each geographic group. Results were distilled into modal sequence (most common order) trajectories, including the variation around the modal sequence. RESULTS: Population variation occurs within and across all geographic groups, especially in later fusing sites. Some sites, such as the acromion and sacral elements, consistently exhibit greater variability. Among outliers from the modal sequence, it is more common for early-fusing sites to fuse late than for later-fusing sites to fuse early. The completing fusion trajectories vary less than the beginning fusion trajectories. DISCUSSION: Despite considerable variation within and among different geographic groups, there are shared commonalities across different samples that can facilitate differentiation of multiple individuals. With fewer outliers, the completing fusion trajectories are potentially of greater practical use in forensic and bioarcheological practice.