Some Aspects on an Asymmetry of Nasal Bones in Toothed Whales

The arrangement of cranial bones in cetaceans in both Odontoceti and Mysticeti has evolved to enable better adaptation to aquatic habitats, and as a result, cetacean skulls differ from those of terrestrial mammals. Their unique skull morphology has been studied to the extent that it is now known Odontoceti have unusual asymmetric cranial bone structures, whereas Mysticeti have symmetric structures similar to other mammals (Miller 1923). Ness (1967) measured the asymmetry of odontocete skulls of 314 specimens across seven families, 26 genera, and 41 species. He revealed the presence of a leftward deviation of midline structures and reported that the point most deviant from the midline axis usually lies on the tangent between both nasal bones. Cranial asymmetry includes both the deviation of the midline suture to the left and enlargement or distortion of the bony elements (Heyning 1989). This phenomenon is known as directional asymmetry, which is unique to Odontoceti among mammals. Cranial asymmetry has been pointed out to play a role in the sophisticated acoustic system characteristic of Odontoceti.