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 years ago. This tiny primate weighed 20–30 grams (0.7–1.1 ounce) and could fit within a human palm. Living mammal species can be identified by the presence of [[sweat gland]]s, including [[Mammary gland|those that are specialized to produce milk]] to nourish their young. In classifying fossils, however, other features must be used, since soft tissue glands and many other features are not visible in fossils. Many traits shared by all living mammals appeared among the earliest members of the group: For the most part, these characteristics were not present in the Triassic ancestors of the mammals. Nearly all mammaliaforms possess an