Pakicetus attocki skeleton
Pakicetidae ("Pakistani whales") is an extinct mammalian family of carnivorous cetaceans that lived during the Early Eocene to Middle Eocene (55.8 mya—40.4 mya) Indo-Pakistan and existed for approximately 15.4 million years. Modern whales evolved from archaic whales such as basilosaurids, which in turn evolved from something like the amphibious ambulocetids, which themselves evolved from something like the land-dwelling pakicetids. All species are known only from a few sites in northern Pakistan, hence the name of the family, in a region which was coastal to the Tethys Sea when the pakicetids lived.
Pakicetid ears had an external auditory meatus and ear ossicles (i.e. incus, malleus, tympanic ring, etcetera) similar to those in living land mammals and most likely used normal land mammal hearing in air. In the pakicetid mandible, the mandibular foramen is small and comparable in size to those of extant land mammals and the acoustic mandibular fat pad characteristic of later whales was obviously not present. The lateral wall of the mandible is also relatively thick in pakicetids, further preventing sound transmission through the jaw. The tympanic bulla in pakicetid ears is similar to those in all cetaceans, with a relatively thin lateral wall and thickened medial part known as the involucrum. However, in contrast to later cetaceans, the tympanic bone makes contact with the periotic bone which is firmly attached to the skull leaving no space for isolating air sinuses, effectively preventing directional hearing in water. Pakicetids most likely used bone conduction for hearing in water.
Ichthyolestes ("fish thief") is an extinct genus of mammal belonging to the family Pakicetidae which were endemic to southern Asia living during the Lutetian stage of the Middle Eocene (48.6 mya—40.4 mya) and existing for approximately 8.4 million years.
Ichthyolestes, the oldest of the pakicetids, had semicircular canals in the inner ear the same size as the living relatives of cetaceans, the artiodactyls, in contrast to the reduced size found in later pakicetids and protocetids, such as Remingtonocetus, Indocetus, and Dorudon, and extant cetaceans.Fossils have been exclusive to northern Pakistan.
Nalacetus is an extinct pakicetid early whale, fossils of which have been found in Lutetian (48.6 to 40.4 million years ago) red beds in Punjab, Pakistan (33.6°N 72.2°E, paleocoordinates 14.3°N 68.3°E). Nalacetus lived in a fresh water environment, was amphibious, and carnivorous. It was considered monophyletic by Cooper, Thewissen & Hussain 2009. It was said to be wolf-sized and one of the earliest forms of the order Cetacea.
Nalacetus is known mostly from dental remains from the Lutetian of the Kala Chitta Hill, Punjab, Pakistan.
In the cheek teeth of Pakicetus, the protocone lobe increases from the first molar to the third. In Nalacetus, in contrast, the protocone lobe is larger in the first molar than in the second.
Pakicetus is a genus of extinct amphibious whale of the family Pakicetidae which was endemic to Pakistan from the Eocene (55.8-40.4 Ma). The vast majority of paleontologists regard it as the most basal whale. 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 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 & 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. It was illustrated on the cover of Science as a semiaquatic, somewhat crocodilelike mammal, diving after fish.
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).
Ichthyolestes Ichtyolestes pinfoldi Nalacetus Nalacetus ratimitus Pakicetus Pakicetus attocki Pakicetus calcis Pakicetus chittas