“…Association between traits can be inferred by comparing the ontogeny (i.e., growth trajectories) of different modules or anatomical parts or by direct test of correlation between traits (Lieberman, ). Based on previous studies on different paranasal sinuses, it is expected that the FS is structurally and developmentally associated with other variables, such as: (a) cranial size (Curtis et al, ; Zollikofer et al, ); (b) dimensions of adjacent structures, for example, nasal size (Butaric & Maddux, ; Butaric, McCarthy, & Broadfield, ; Holton, Yokley, & Butaric, ; Maddux & Butaric, 2017); and c) bone thickness and thickness of the glabellar region, as the bony environment provides the space to enable/constrain FS expansion (Curtis et al, ; Farke, ; Vinyard & Smith, ). The ontogenetic analysis of the FS in relation to these structures provides insight into the underlying mechanisms (e.g., the opportunistic pneumatization hypothesis) that shape the observed morphological variation among populations and species (Prossinger, ).…”