Marine mammals are found in several lineages among the more than 6000 species of mam-mals (all belonging to the class Mammalia). Not all marine mammals are closely related. Together they form a habitat-based group of animals associated with water. Mammals evolved on land some 250 million years ago. At least seven separate clades of mammals have returned independently to water, where they have adapted to aquatic habitats. Despite huge differences between species, terrestrial and marine mammals share many common features. Cetaceans (whales) evolved from ter-restrial ancestors which were hoofed ani-mals (ungulates, belonging to the order Artiodactyla which also contain hippopot-amus, pigs and deer) more than 50 million years ago, during the geological epoch called the Eocene. The closest now-living terrestrial relative of cetaceans is the hippopotamus and the ruminants, including cattle. One of the earliest known cetaceans is Pakicetus(meaning ‘the cetacean from Pakistan’). Pakicetus was mostly terrestrial but walked and most probably hunted in shallow waters. It looked like a small dog with a long snout, incisors and hooves.