The meaning of the shape of the frontal bone, facial retraction and prognathism for the degree of gracilisation of the supraorbital region in Homo sapiens

Although the marked convexity of the frontal bone and retraction of the facial skeleton under the anterior part of the cranial base are commonly regarded as traits significantly influencing the gracilisation of the supraorbital region (a decrease in the degree of robusticity) in the Homo sapiens crania, identification of which of these two traits is more important for this phenomenon has not yet been investigated. Thus, the main aim of this study is to establish whether the frontal inclination angle (reflecting the mid-sagittal shape of the frontal bone) or the facial retraction angle (describing the grade of facial retraction) is more strongly related to the degree of gracilisation in the supraorbital region in a large sample of geographically diverse modern adult human crania (n = 250). Earlier studies based on small modern Homo sapiens cranial samples (derived from one population) demonstrated a lack of a relationship between the facial angle (reflecting the prognathism of the lower face) and the supraorbital region’s expression. In this study, we also investigated whether there was no relationship between these traits in the analysed large modern human cranial sample. Spearman’s rank correlations and models concerning partial rank correlations (including traits such as sex, geographic origin, size of the facial skeleton and size of the neurocranium to eliminate their influence on the examined relationships) were used. The results indicate a stronger relationship of the degree of gracilisation in the supraorbital region with the frontal inclination angle than with the angle of facial retraction (without the influence of the other considered factors). No significant relationship between supraorbital region expression and prognathism was established. The importance of the obtained results was discussed, including the evolutionary perspective and the occurrence of significant interpopulation differences in the gracilisation of the supraorbital region in modern humans.