The term whale can refer to all cetaceans, to just the larger ones, or only to members of particular families within the order Cetacea. The last definition is the one followed here. Whales are those cetaceans which are neither dolphins (i.e. members of the families Delphinidae or Platanistoidea) nor porpoises. This can lead to some confusion because Orcas ( Killer Whales ) and Pilot whales have whale in their name, but they are dolphins for the purpose of classification. All cetaceans, including whales, dolphins and porpoises, are descendants of land-living mammals of the Artiodactyl order (even-toed ungulate animals). Both cetaceans and artiodactyl are now classified under the super-order Cetartiodactyla which includes both whales and hippos. In fact, whales are the closest living relatives of hippos; they evolved from a common ancestor at around 54 million years ago. Whales entered the water roughly 50 million years ago. Cetaceans are divided into two suborders: * The baleen whales are characterized by baleen, a sieve-like structure in the upper jaw made of keratin, which they use to filter plankton from the water. They are the largest species of whale. * The toothed whales have teeth and prey on fish, squid, or both. An outstanding ability of this group is to sense their surrounding environment through echolocation.