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 the early [[Permian]]. Nonmammalian synapsids are sometimes called "mammal-like reptiles". [[Therapsid]]s, a group of synapsids, descended from [[pelycosaur]]s in the Middle Permian, about 265 million years ago, and became the dominant land vertebrates. They differ from basal [[Eupelycosauria|eupelycosaurs]] in several features of the skull and jaws, including: larger skulls and [[incisor]]s which are equal in size in therapsids, but not for eupelycosaurs. The therapsid lineage leading to mammals went through a series of stages, beginning with animals that were very similar to their pelycosaur ancestors and ending with [[probainognathia]]n [[cynodont]]s, some of which could easily be mistaken for mammals. Those