Cetacea , (from Latin cetus "large sea creature" and Greek ketos "sea-monster") are a widely distributed and diverse infraorder of carnivorous, aquatic, marine mammals. They comprise the families Balaenidae (right whales), Balaenoptera (rorqual), Eschrichtiidae (the gray whale), Delphinidae (oceanic dolphins), Monodontidae (Arctic whales), Phocoenidae (porpoises), Physeteridae (the sperm whale), Kogiidae (lesser sperm whales), Platanistidae (Old World river dolphins), Iniidae (New World river dolphins), Pontoporiidae (the La plata dolphin), and Ziphidae (beaked whales). There are currently 88 species of cetacean. While cetaceans were historically thought to have descended from mesonychids, molecular evidence supports them as descendants of Artiodactyla (even-toed ungulates). Cetaceans belong to the order Cetartiodactyla (derived from Cetacea and Artiodactyla) and their closest living relatives are hippopotamuses, having diverged about 60 million years ago.