Temporal range: Ypresian
|Cast of P. attocki, ROM|
Gingerich & Russell 1981
The first fossil found consisted of an incomplete skull with a skull cap and a broken mandible with some teeth. Based on the detail of the teeth, the molars suggest that the animal could rend and tear flesh. Wear, in the form of scrapes on the molars indicated that Pakicetus ground its teeth as it chewed its food. Because of the toothwear, Pakicetus is thought to ate fish and small animals. The teeth also suggest that Pakicetus had herbivorous and omnivorous ancestors. 
Pakicetus was originally described as being a, then later, as being descended from a mesonychid, but Gingerich & Russell 1981 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.
Possible semi-aquatic nature
Somewhat more complete skeletal remains were discovered in 2001, prompting the view that Pakicetus was primarily a land animal about the size of a wolf, and very similar in form to the related mesonychids. Thewissen et al. 2001 wrote that "Pakicetids were terrestrial mammals, no more amphibious than a tapir."
However, Thewissen et al. 2009 argued that "the orbits ... of these cetaceans were located close together on top of the skull, as is common in aquatic animals that live in water but look at emerged objects. Just like Indohyus, limb bones of pakicetids are osteosclerotic, also suggestive of aquatic habitat" (since heavy bones provide ballast). "This peculiarity could indicate that Pakicetus could stand in water, almost totally immersed, without losing visual contact with the air."
The Pakicetus skeleton reveals several details regarding the creature's unique senses, and provides a newfound ancestral link between terrestrial and aquatic animals. As previously mentioned, the Pakicetus' upward-facing eye placement was a significant indication of its habitat. Even more so, however, was its auditory abilities. Like all other cetaceans, Pakicetus had a thickened skull bone known as the auditory bulla, which was specialized for underwater hearing. Cetaceans also all categorically exhibit a large mandibular foramen within the lower jaw, which holds a fat pack and extends towards the ear, both of which are also associated with underwater hearing. "Pakicetus is the only cetacean in which the mandibular foramen is small, as is the case in all terrestrial animals. It thus lacked the fat pad, and sounds reached its eardrum following the external auditory meatus as in terrestrial mammals. Thus the hearing mechanism of Pakicetus is the only known intermediate between that of land mammals and aquatic cetaceans." With both the auditory and visual senses in mind, as well as the typical diet of Pakicetus, one might assume the creature was able to attack both aquatic and terrestrial prey from a low vantage point.
The first fossils were uncovered in Pakistan, hence their name. They were found within the Kuldana formation located in northern Pakistan and were dated as early to early-middle Eocene in age. The fossils came out of red terrigenous sediments bounded largely by shallow marine deposits typical of coastal environments caused by the Tethys Sea.  Speculation is that many major marine banks flourished with the presence of this prehistoric whale. According to the location of fossil findings, they preferred a shallow habitat that neighbored decent sized land. Assortments of limestone, dolomite, mudstone and other varieties of different colored sands has been predicted to be a favorable habitat for such.
- Pakicetus in the Paleobiology Database. Retrieved June 2013.
- Gingerich & Russell 1981
- Gingerich et al. 1983, Cover
- Thewissen et al. 2001, p. 278
- Thewissen et al. 2009, p. 277
- de Muizon 2009
- Geisler, Jonathan; Ho, Melody. "Pakicetus spp.". New York Institute of Technology. Retrieved October 2013.
- Thewissen & Hussain 1993
- Gingerich & Russell 1981, p. 239
- Cooper, Thewissen & Hussain 2009, p. 1291
- Gingerich 2003, p. 645
- Polly, Paul D. "Pakicetus (fossil Mammal Genus)". Encyclopedia Britannica Online. Retrieved 24 Oct. 2013.
- "New middle Eocene archaeocetes (Cetacea: Mammalia) from the Kuldana Formation of northern Pakistan". Journal of Vertebrate Paleontology 29 (4): 1289–1299. 2009. doi:10.1671/039.029.0423.
- de Muizon, Christian (March 2009). "L’origine et l’histoire évolutive des Cétacés/Origin and evolutionary history of cetaceans". Comptes Rendus Palevol 8 (2–3): 295–309. doi:10.1016/j.crpv.2008.07.002.
- Cooper, Lisa Noelle; Thewissen, J. G. M.; Hussain, S. T. (2009). "New middle Eocene archaeocetes (Cetacea: Mammalia) from the Kuldana Formation of northern Pakistan". Journal of vertebrate paleontology 104 (4): 1289–99. doi:10.1671/039.029.0423. OCLC 506008976.
- Gingerich, Philip D.; Wells, N. A.; Russell, Donald E.; Shah, S. M. Ibrahim (April 22 1983). "Origin of Whales in Epicontinental Remnant Seas: New Evidence from the Early Eocene of Pakistan". Science 220 (4595): 403–6. doi:10.1126/science.220.4595.403. PMID 17831411. Retrieved June 2013.
- Gingerich, Philip D.; Russell, Donald E. (1981). "Pakicetus inachus, A New Archaeocete (Mammalia, Cetacea) from the Early-Middle Eocene Kuldana Formation of Kohat (Pakistan)". Contributions from the Museum of Paleontology, The Museum of Michigan 25 (11): 235–246. OCLC 742729300. Retrieved February 2013.
- Gingerich, Philip D. (2003). "Stratigraphic and micropaleontological constraints on the middle Eocene age of the mammal-bearing Kuldana Formation of Pakistan". Journal of Vertebrate Paleontology 23 (3): 643–651. doi:10.1671/2409.
- Thewissen, J. G. M.; Williams, E. M.; Roe, L. J.; Hussain, S. T. (2001). "Skeletons of terrestrial cetaceans and the relationship of whales to artiodactyls". Nature 413 (6853): 277–281. doi:10.1038/35095005. PMID 11565023. Retrieved June 2013.
- Thewissen, J. G. M.; Cooper, Lisa Noelle; George, John C.; Bajpai, Sunil (2009). "From Land to Water: the Origin of Whales, Dolphins, and Porpoises". Evolution: Education and Outreach 2 (2): 272–288. doi:10.1007/s12052-009-0135-2. Retrieved June 2013.
- Thewissen, J.G.M.; Hussain, S.T. (1993). "Origin Of Underwater Hearing In Whales". Nature 361 (6411): 444–445. doi:10.1038/361444a0.
- West, Robert M (1980). "Middle Eocene large mammal assemblage with Tethyan affinities, Ganda Kas region, Pakistan". Journal of Paleontology 54 (3): 508–533. JSTOR 1304193. OCLC 4899161959.