Cetaceans are divided into two suborders:
The largest suborder, Mysticeti (baleen whales), is characterised by the presence of baleen, a sieve-like structure in the upper jaw made of keratin, which it uses to filter plankton from the water.
Odontoceti (toothed whales) bear sharp teeth for hunting. Odontoceti includes dolphins and porpoises. If they were not considered to be whales, this would mean that the informal grouping 'whale' is not a clade.
Cetaceans and artiodactyls now are classified under the super-order Cetartiodactyla, which includes both whales and hippopotami. Whales are the hippopotamus's closest living relatives.[4]