The first fossil , a lone skull , was thought to be a mesonychid , but Gingerich and Russell recognized it as an early cetacean from characteristic features of the inner ear , found only in cetaceans : the large auditory bulla is formed from the ectotympanic bone only . It was thought to be from a mesonychid , but Gingerich and Russell recognized it as an early cetacean from characteristic features of the inner ear , found only in cetaceans : the large auditory bulla is formed from the ectotympanic bone only .