There are characteristics unique to cetaceans. The first one is the posterior process that is enlarged from the posterior crus, forming the primary bullar articulation to the basicranium. The second defining characteristic is that the sigmoid process is enlarged. The third characteristic is a conical apophysis that is modified from the ectotympanic ring. Then there is the conical tympani ligament, derived from the tympanic membrane. Lastly, the involucrum formed from the pachyosteosclerosis of the bulla.  These unique traits evolved overtime, from the terrestrial Pakicetidae to the crown whales today.