 Although they look very much like modern cetaceans , basilosaurids lacked the ' melon organ ' that allows toothed whales to use echolocation . They had small brains ; this suggests they were solitary and did not have the complex social structures of some modern cetaceans . The mandibular foramen of basilosaurids covered the entire depth of the lower jaw as in modern cetaceans . Their orbits faced laterally , and the nasal opening had moved even higher up the snout , closer to the position of the blowhole in modern cetaceans . Furthermore , their ear structures were functionally modern , with the insertion of air @-@ filled sinuses between ear and skull . Unlike modern cetaceans , basilosaurids retained a large external auditory meatus . 
