 Pakicetids are classified as cetaceans mainly due to the structure of the auditory bulla ( ear bone ) , which is formed only from the ectotympanic bone . The shape of the ear region in pakicetids is highly unusual and the skull is cetacean @-@ like , although a blowhole is still absent at this stage . The jawbone of pakicetids also lacks the enlarged space ( mandibular foramen ) that is filled with fat or oil , which is used in receiving underwater sound in modern cetaceans . They have dorsal orbits ( eye sockets facing up ) , which are similar to crocodiles . This eye placement helps submerged predators observe potential prey above the water . According to a 2009 study , the teeth of pakicetids also resemble the teeth of fossil whales , being less like a dog 's incisors , and having serrated triangular teeth , which is another link to more modern cetaceans . It was initially thought that the ears of pakicetids were adapted for underwater hearing , but , as would be expected from the anatomy of the rest of this creature , the ears of pakicetids are specialized for hearing on land . However , pakicetids were able to listen underwater by using enhanced bone conduction , rather than depending on the tympanic membrane like other land mammals . This method of hearing did not give directional hearing underwater . 
