Meeting Abstract
With adaptations for an aquatic lifestyle, toothed whales have blowholes and nasal passages that bear little resemblance to those of their land relatives. We used CT scanning and histological sectioning to understand the formation of the respiratory tract during development, using a complete ontogenetic series of dolphins of the genus Stenella. We found that the nasal placode develops just superior to the upper lip, similar to land mammals. In the first few weeks of development, the nasal opening shifts caudal onto the forehead. At Carnegie stage 16-17 (around week 5-6 of gestation), dolphin embryos display two external nares. Soon thereafter, these merge to form one blowhole. Around this time (Carnegie 16-19), asymmetry of the nasal opening arises, with the right side elevated above the surface of the forehead. Around Carnegie 20 (roughly 3 months), the single blowhole is oriented obliquely on the forehead, with its right side more dorsal than the left. Even at Carnegie 17, the larynx displays the typical cetacean shape, with spout-shaped epiglottic and corniculate cartilages that project towards but not into the nasopharyngeal duct. Around Carnegie 17, the diverticula of the nasal passages develop, particularly the premaxillary sacs, with few signs of the other air sacs involved in sound production. Understanding the development of the respiratory tract informs the interpretation of nasal passages in Eocene whales and baleen whales. A comparison with Eocene whales indicates that the nasopharyngeal duct anatomy is very different from that of modern dolphins, suggesting differences in the function of the air and food pathways.
