Enamel structure in odontocete whales.

With regard to the enamel structure of mammals, a large number of studies have been reported in the past. Of them, however, the enamel structure of odontocetes has not yet been sufficiently elucidated. The author therefore observed the enamel structure of 11 species in 7 families of living odontocetes. A clear prism structure in the enamel is noted in delphinids and Pontoporia blainvillei. Neophocaena phocaenoides has a very simple-structured prism, but even this structure is obvious only in the deep layer of the enamel, disappearing gradually from the mid layer to surface layer. The prism pattern of delphinids differs significantly depending on the site of the enamel; that of Pontoporia shows as a whole pattern 1. On the other hand, the enamel of Physeter catodon, Berardius bairdi, Phocoena phocoena, Phocoenoides dalli and Delphinapterus leucus is prismless. The enamel of Physeter and Phocoena shows pseudoprisms; that of Phocoenoides contains enamel tubuli. The enamel of Berardius and Delphinapterus is 3 to 8 micron thick, which is extremely thin for mammalian enamel. No enamel was noted in Monodon monoceros teeth. The enamel structure of living odontocetes is thus very variable. Several characteristic structures having been present during the evolutionary course of this tissue are still present in some of them. As the results of comparative histologic study, it is considered that the variable enamel structure of living odontocetes is a secondary phenomenon produced during the degenerative history of the enamel.