Vascularization of the gray whale palate (Cetacea, Mysticeti, Eschrichtius robustus): soft tissue evidence for an alveolar source of blood to baleen.

The origin of baleen in mysticetes heralded a major transition during cetacean evolution. Extant mysticetes are edentulous in adulthood, but rudimentary teeth develop in utero within open maxillary and mandibular alveolar grooves. The teeth are resorbed prenatally and the alveolar grooves close as baleen germ develops. Arteries supplying blood to highly vascularized epithelial tissue from which baleen develops pass through lateral nutrient foramina in the area of the embryonic alveolar grooves and rudimentary teeth. Those vessels are hypothesized to be branches of the superior alveolar artery, but branches of the greater palatine arteries may play a role in the baleen vascularization. Through a combination of latex injection, CT, and traditional dissection of the palate of a neonatal gray whale (Eschrichtius robustus), we confirm that the baleen receives blood from vessels within the superior alveolar canal via the lateral foramina. The greater palatine artery is restricted to its own passage with no connections to the baleen. This study has implications for the presence of baleen in extinct taxa by identifying the vessels and bony canals that supply blood to the epithelium from which baleen develops. The results indicate that the lateral foramina in edentulous mysticete fossils are bony correlates for the presence of baleen, and the results can be used to help identify bony canals and foramina that have been used to reconstruct baleen in extinct mysticetes that retained teeth in adulthood. Further comparisons are made with mammals that also possess oral keratin structures, including ruminants, ornithorhynchid monotremes, and sirenians.