Development of tooth structure in aquatic mammals

In this study, enamel structures such as the Hunter-Schreger bands and enamel prisms in the middle layer are compared among completely or semi aquatic mammals. Cetacea, Pinnipedia, Sirenia have become completely adapted for life in the water, but Hippopotamus (Ungulate) and Enhydra (Carnivora) are semi aquatic. The fossil mammal Desmostylia was concluded to be to a considerable degree adapted for an aquatic habit. Cetacean evolution suggests that the enamel structures have become reduced and more and more simple along with the degeneration of tooth form. The enamel structures of Enhydra and Phoca may show a halfway stage in this degeneration process. The enamel patterns of the Miocene Sirenian, Halitherium, show a slightly more complex course of enamel prisms, which may be the original pattern of the Hunter-Schreger bands which had arisen along with the molar evolution. However, this evolutionary line might disappear on adoption of completely aquatic life. In the Desmostylid, the structure of the molars evolved to become more and more complex and enamel thickness rose to about 10 mm, which is the thickest in mammalia. This type of change arose only in the terrestrial and leaf eating species of aquatic Ungulates such as Hippopotamus. For that reason, these completely aquatic mammals might come on the shore only when they wanted a meal of hard and fibrous plants.