Sirenian and Other Marine Mammals: Evolution and Systematics

The mammalian order Sirenia, or sea cows, includes two extant families: the Trichechidae (manatees) and the Dugongidae (the dugong). Manatees include three living species and are known from the early Miocene (15 Ma) to the recent time in the New World tropics. The dugong is represented by a single extant species: Dugong dugon of the Indo-Pacific. Sirenians are unique among living marine mammals in having a strictly herbivorous diet, which is reflected in the morphology of their teeth and digestive system. The monophyly of sirenians is widely accepted, and elephants are considered their closest living relatives. Sirenians, elephants, and extinct desmostylians form a monophyletic clade, the Tethytheria, which is part of a larger, diverse mammal clade: the Afrotheria. An extinct lineage of dugongids led to the recently extinct Steller's sea cow, which was cold-adapted for life in the Bering Sea, in contrast to other members of this lineage distributed in tropical or subtropical waters. The hippopotamus-like desmostylians have the distinction of composing the only extinct order of marine mammals. The large extinct bear-like carnivoran Kolponomos is now recognized as more closely related to amphicynodontine ursids and pinnipedimorphs rather than its previous allocation to the raccoon family.