Temporal range: Early Eocene
Gingerich & Russell, 1981
Pakicetus is a genus of extinct cetaceans found in the early Eocene (55.8 ± 0.2 - 33.9 ± 0.1 Ma) of Pakistan, hence their name. The strata where the fossils were found was then part of the coast of the Tethys Sea.
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. This suggests that it is a transitional species between extinct land mammals and modern cetaceans.
Complete skeletons were discovered in 2001, revealing that Pakicetus was primarily a land animal, about the size of a wolf, and very similar in form to the related mesonychids.