The meaning of emptiness: Sinuses and sacs from land to sea

Abstract

The processes and patterns of skull pneumatization have intrigued anatomists for over two thousand years. The paranasal sinuses, in particular, have been observed in an array of animals including: amphibians, reptiles, birds, and terrestrial mammals. Sinus function has eluded investigators, leading some to conclude that they may be non‐functional in nature (vestigial? spandrels?). This is supported by their reduction/absence in some derived vertebrates (e.g., marine mammals). Investigators searching for the purpose of the paranasal sinuses have classified them in three broad categories of function/existence: 1) architectural ‐ e.g., distribution of weight/musculoskeletal forces, 2) physiological ‐ e.g., respiratory/vocal functions, or 3) vestigial ‐ the biological roles these spaces had in their phylogenetic history is no longer relevant. In diving mammals, the constraints of changing pressures made it impractical, and even dangerous, to maintain rigid sinuses with fixed volumes. However, sinus disappearance in diving mammals does not seem merely related to the gradual reduction of a vestigial structure. Rather, many terrestrial sinus functions were preserved and transferred to a novel set of structures: flexible walled, extracranial diverticulae called air sacs. The preservation of air spaces indicates that while the sinuses/air sacs may be "empty," they are still full of function.