There are some 120 extant species of marine mammals, generally subdivided into the five groups bold-faced below.Order Sirenia: Sirenians
family Trichechidae: manatees (3 species)
family Dugongidae: dugong (1 species)
Order Cetacea: Cetaceans
Suborder Mysticeti: Baleen whales (14 or 15 species)
Suborder Odontoceti: Toothed whales (around 73 species)
Order Carnivora,
superfamily Pinnipedia
family Phocidae: true seals (around 20 species)
family Otariidae: eared seals (around 16 species)
family Odobenidae: walrus (1 species)
family Mustelidae
sea otter (Enhydra lutris)
marine otter (Lontra felina)
family Ursidae
polar bear (Ursus maritimus)
The sirenians and cetaceans are thought to be descendent from an ungulate ancestor, while the pinnipeds, otters and polar bear are descended from a caniform ancestor. The morphological similarities between these diverse groups are a result of convergent and parallel evolution.
