Marine mammal Marine mammal Marine mammals are aquatic mammals that rely on the ocean and other marine ecosystems for their existence. They include animals such as seals, whales, manatees, sea otters and polar bears. They do not represent a distinct taxon or systematic grouping, but rather have a polyphyletic relation due to convergent evolution, as in they do not have an immediate common ancestor. They are also unified by their reliance on the marine environment for feeding. Marine mammal adaptation to an aquatic lifestyle varies considerably between species. Both cetaceans and sirenians are fully aquatic and therefore are obligate water dwellers. Seals
Mammal determination system]] different from that of most other mammals. In particular, the [[sex chromosome]]s of a platypus are more like those of a chicken than those of a therian mammal. Viviparous mammals are in the subclass Theria; those living today are in the marsupial and placental infraclasses. Marsupials have a short [[gestation]] period, typically shorter than its [[estrous cycle]] and gives birth to an undeveloped newborn that then undergoes further development; in many species, this takes place within a pouch-like sac, the [[Pouch (marsupial)|marsupium]], located in the front of the mother's [[abdomen]]. This is the [[Symplesiomorphy|plesiomorphic]] condition among viviparous mammals;