Whales are a widely distributed and diverse group of fully aquatic placental marine mammals. They are an informal grouping within the infraorder Cetacea, which usually excludes dolphins and porpoises. The two parvorders of whales, baleen whales (Mysticeti) and toothed whales (Odontoceti), are thought to have had their last common ancestor around 34 million years ago. Whales consist of eight extant families: Balaenopteridae (the rorquals), Baloenidae (right whales), Cetotheriidae (pygmy right whales), Eschrichtiidae (grey whales), Physeteridae (sperm whales), Monodontidae (belugas and narwhals) and Ziphiidae (beaked whales).