Function and evolution of the cranial sinuses in bovid mammals and ceratopsian dinosaurs.

摘要

The paranasal sinuses, air-filled spaces within the skull originating as diverticula from the nasal cavity, are the object of numerous functional hypotheses. For instance, it has been hypothesized that enlarged frontal sinuses in bovids (the clade of horned artiodactyl mammals including goats, cattle, antelope and their relatives) and supracranial sinuses in ceratopsians (the clade of horned dinosaurs including Triceratops) function as shock absorbers to protect the brain during horn-to-horn combat. At present, the most commonly accepted hypothesis indicates that the sinuses in all terrestrial vertebrates are functionless structures resulting from the removal of mechanically unnecessary bone ("opportunistic pneumatization"). Yet, this and other hypotheses remain largely untested.;Finite element modeling (FEM) was used to examine the functional role of the frontal sinuses in domesticated goats. Models of the skull, with varying frontal bone and sinus morphology, were loaded under simulated head-butting conditions. It was found that the sinuses are only moderately effective as shock absorbers, are poorly placed for protecting the brain from blows to the horns, and are located in areas of bone under low stress.;Frontal sinuses were measured and described for 63 species of Bovidae, in order to document the morphology and variation of the frontal sinus in this clade. Character optimization suggests that frontal sinuses were present at the origin of Bovidae, and secondarily reduced or lost at least six different times. No statistically significant link was found between head-butting behavior and sinus morphology (size or structural complexity). Partial correlations found a significant correlation between the volume of the frontal sinus and the size of the frontal bone, but not between the volume of the frontal sinus and horn or skull size. Both the FEM and comparative analyses were interpreted to be partially consistent with the hypothesis of frontal sinuses resulting from opportunistic pneumatization of structurally unnecessary bone.;Based on the results in bovids, the hypothesis that sinuses acted as shock absorbers in ceratopsians is weakened. In this clade, unlike in Bovidae, the sinuses form through a secondary roofing of the skull in conjunction with excavation of bone. The development of a closed sinus from an open depression was probably associated with an increase in skull and horn size in order to maintain the structural integrity of the skull, as well as an anatomical reorganization of the ceratopsian skull which resulted in a thickening of the skull roof. Although a pneumatic origin cannot be demonstrated irrefutably in ceratopsians, any air source would have had to enter the dorsum of the skull via the dorsotemporal fenestrae.