 The two modern parvorders of cetaceans – Mysticeti ( baleen whales ) and Odontoceti ( toothed whales ) – are thought to have separated from each other , and the archaeocetes , around 34 million years ago in a second cetacean radiation , the first occurring with the archaeocetes . The adaptation of echolocation in toothed whales distinguishes them from fully aquatic archaeocetes and early baleen whales . The presence of baleen in baleen whales occurred gradually , with earlier varieties having very little baleen , and their size is linked to baleen dependence ( and subsequent increase in filter feeding ) . 
