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 metatherian "Sinodelphys", both dated to 125 million years ago. [[Epipubic]] bones, a feature that strongly influenced the reproduction of most mammal clades, are first found in [[Tritylodontidae]], suggesting that it is a synapomorphy between them and [[mammaliformes]]. They are omnipresent in non-placental mammaliformes, though "[[Megazostrodon]]" and "[[Erythrotherium]]" appear to have lacked them. It has been suggested that the original function of [[lactation]] ([[milk]] production) was to keep eggs moist. Much of the argument is based on monotremes, the egg-laying mammals. Therian mammals took over the medium- to large-sized ecological niches in the [[Cenozoic]], after the [[Cretaceous–Paleogene extinction event]] approximately