Prehistoric Whales

                                                   

                   

                         Pakicetus     

                

                                   

Pakicetus is a genus of extinct terrestrial carnivorous mammal of the family Pakicetidae

-which was endemic to Pakistan from the Eocene (55.8 ± 0.2—40 ± 0.1 Ma). Pakicetus existed for approximately 15.8 million years. Many paleontologists regard it as a close relative to the direct ancestors of modern day whales.

 Pakicetus was assigned to Protocetidae by Gingerich and Russell (1981), Carroll (1988) and Benton (1993). Then to Pakicetinae by Gingerich and Russell (1990) and McKenna and Bell (1997); and to Pakicetidae by Thewissen and Hussain (1998), Thewissen et al. (2001), Thewissen et al. (2001), Geisler and Sanders (2003), McLeod and Barnes (2008) and Uhen (2010).

 The first fossils were uncovered in Pakistan, hence their name. The strata of western Pakistan where the fossils were found was then the coastal region of the Tethys Sea. The first fossil found of the creature consisted of an incomplete skull with a skull cap and a broken mandible with some teeth. 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. This suggests that it is a transitional species between extinct land mammals and modern cetaceans. It was restorated on the cover of Science as a semiaquatic, somewhat crocodile like mammal, diving after fish.

 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. In 2001, J. G. M. Thewissen and colleagues wrote that "Pakicetids were terrestrial mammals, no more amphibious than a tapir."
However, in 2009 Thewissen et al 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"