UCLA

Mammal skulls contain up to four mucosal-lined, air-filled cavities called paranasal sinuses within the bones surrounding the nasal chamber, including the maxilla, ethmoid, sphenoid, and frontal. Paranasal sinuses are highly variable in presence and morphology among mammals, and their function is not well understood due to the fact that they are hidden within the skull and inaccessible without use of destructive methods. The leading hypothesis to explain sinus function is that they opportunistically form where bone is mechanically unnecessary. Sinuses may also help dissipate stress more evenly across the skull during feeding and other behaviors. To test these hypotheses, I conducted the first quantitative and comparative investigation of how frontal sinus morphological disparity relates to skull morphology, ecology and diet in mammalian carnivores. To do so, I used non-destructive CT scans and applied a novel technique to quantify the three-dimensional shape of sinuses. Cranial shape, body size, diet, and ecology vary markedly within Carnivora, with many examples of convergence, making them an ideal framework within which to examine sinus function. I quantified frontal sinus morphology for fifty-six carnivore species, including a large intraspecific sample of coyotes (Canis latrans) with associated age and diet information. Results support the hypothesis that frontal sinuses from where bone is mechanically unnecessary, but several taxa lacked frontal sinuses, suggesting that there may be phylogenetic constraints on which taxa can develop frontal sinuses. Among Carnivora with frontal sinuses, sinus morphology was strongly correlated with the size and shape of the frontal bone, and was also correlated with allometric differences in skull shape between families that relate to biomechanical function. Skull shape disparity related to ecology also appears to affect frontal sinus morphology. Dorsal flattening of the skull roof in aquatic and fossorial carnivores was associated with reduced or absent frontal sinuses. Large, anteriorly oriented eyes and foreshortened snouts of arboreal species also appear to limit space where a sinus can form. Intraspecific variation in sinus shape suggests frontal sinus morphology is affected by diet-related skull utility, and that frontal sinus morphology can vary throughout an organism's life and be modified to improve skull performance.