Evolution and functional morphology of the frontal sinuses in Bovidae (Mammalia: Artiodactyla), and implications for the evolution of cranial pneumaticity

Abstract

The function and evolution of paranasal pneumaticity remains elusive, in part because of limited sampling and description. Here, the frontal sinuses from 62 species of bovids were investigated using X-ray computed tomography. This survey revealed hitherto undescribed diversity in the morphology of this sinus, and suggests that it was probably present in the common ancestor of Bovidae. Among extant bovids, the frontal sinuses were lost or reduced to recesses at least six different times. Quantitative analyses, when accounting for phylogeny using phylogenetically independent contrasts, did not find any link between the size or complexity of the frontal sinus and head-to-head ramming behaviour. Other analyses indicated that frontal sinus size was correlated most closely with the size of the frontal bone itself, rather than with the overall skull size or horn size. These results may be partially consistent with the hypothesis of sinuses being the result of ‘opportunistic pneumatization’, in which sinus size depends on the quantity of bone available for pneumatization as well as the mechanical demands placed on the skull. Additional evidence also indicates a strong phylogenetic correlation with sinus morphology, particularly with regard to the presence of paranasal diverticula, as well as the ability of sinuses to cross sutural boundaries.