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 work on the systematics, relationships and occurrences of all mammal taxa, living and extinct, down through the rank of genus, though molecular genetic data challenge several of the higher level groupings. The authors worked together as [[paleontologist]]s at the [[American Museum of Natural History]], [[New York City|New York]]. McKenna inherited the project from Simpson and, with Bell, constructed a completely updated hierarchical system, covering living and extinct taxa that reflects the historical genealogy of Mammalia. [[Extinct]] groups are represented by a [[Dagger (typography)|dagger]] (†). Class Mammalia Molecular studies based on [[DNA]] analysis have suggested new relationships among mammal families over