Pakicetus
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''Pakicetus'' is an extinct
genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure ...
of amphibious
cetacea Cetaceans (from la, Cetus (mythology), cetus, lit=whale, from grc, κῆτος, translit=Cetus (mythology), kētos, lit = huge fish, sea monster) are aquatic mammals constituting the infraorder Cetacea (). Key characteristics are their fully aq ...

cetacea
n of the
family In , family (from la, familia) is a of people related either by (by recognized birth) or (by marriage or other relationship). The purpose of families is to maintain the well-being of its members and of society. Ideally, families would off ...
Pakicetidae Pakicetidae ("Pakistani whales") is an extinct Family (biology), family of Archaeoceti (early whales) that lived during the Early Eocene in Pakistan. Description described the first pakicetid, ''Ichthyolestes'', but at the time they did not reco ...
, which was endemic to Pakistan during the Eocene, about 50 million years ago. It was an animal rather like a wolf, about to long, and lived in and around water where it ate fish and small animals. The vast majority of paleontologists regard it as the most Basal (phylogenetics), basal whale, representing a transitional stage between land mammals and whales. It belongs to the even-toed ungulates with the closest living non-cetacean relative being the hippopotamus.


Description

Based on the skull sizes of specimens, and to a lesser extent on composite skeletons, species of ''Pakicetus'' are thought to have been to in length. ''Pakicetus'' looked very different from modern cetaceans, and its body shape more resembled those of land-dwelling hoofed mammals. Unlike all later cetaceans, it had four fully functional long legs. ''Pakicetus'' had a long snout; a typical complement of teeth that included incisors, canines, premolars, and molars; a distinct and flexible neck; and a very long and robust tail. As in most land mammals, the nose was at the tip of the snout. Reconstructions of pakicetids that followed the discovery of composite skeletons often depicted them with fur; however, given their relatively close relationships with Hippopotamus, hippos, they may have had sparse body hair. 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 tooth wear, ''Pakicetus'' is thought to have eaten fish and small animals. The teeth also suggest that ''Pakicetus'' had herbivorous and omnivorous ancestors.


Palaeobiology


Possible semi-aquatic nature

It was illustrated on the cover of ''Science (journal), Science'' as a semiaquatic, vaguely 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. wrote that "Pakicetids were terrestrial mammals, no more amphibious than a tapir." However, 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 Osteosclerosis, 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."


Sensory capabilities

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 that the creature was able to attack both aquatic and terrestrial prey from a low vantage point.


History of discovery

The first fossil, a skull fragment of ''P. inachus'', was found in 1981 in Pakistan. Subsequent fossils of ''Pakicetus'' were also found in Pakistan, hence the generic name ''Pakicetus''. The fossils were found in the Kala Chitta Range, Kuldana Formation in Kohat in northern Pakistan and were dated as Ypresian, early to Lutetian, 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 Ocean. Speculation is that many major marine banks flourished with the presence of this prehistoric whale. According to the location of fossil findings, the animals preferred a shallow habitat that neighbored decent-sized land. Assortments of limestone, dolomite, stone mud and other varieties of different coloured sands have been predicted to be a favourable habitat for ''Pakicetus''. During the Eocene, Pakistan was a coastal region of Eurasia, and therefore an ideal habitat for the evolution and diversification of the
Pakicetidae Pakicetidae ("Pakistani whales") is an extinct Family (biology), family of Archaeoceti (early whales) that lived during the Early Eocene in Pakistan. Description described the first pakicetid, ''Ichthyolestes'', but at the time they did not reco ...
.


Classification

''Pakicetus'' was classified as an early cetacean due to characteristic features of the inner ear found only in cetaceans (namely, the large auditory bulla is formed from the ectotympanic bone only). It was recognized as the earliest member of the family
Pakicetidae Pakicetidae ("Pakistani whales") is an extinct Family (biology), family of Archaeoceti (early whales) that lived during the Early Eocene in Pakistan. Description described the first pakicetid, ''Ichthyolestes'', but at the time they did not reco ...
. Thus, ''Pakicetus'' represents a transitional taxon between extinct land mammals and modern cetaceans. believed ''Pakicetus'' to be a mesonychid. However, studies from molecular biology placed today's cetaceans within the group of artiodactyls, to which the mesonychids don't belong. In 2001, fossils of ancient whales were found that featured an ankle bone, the Talus bone, astragalus, with a "double pulley" shape characteristic of artiodactyls. The redescription of the primitive, semi-aquatic small deer-like artiodactyl ''Indohyus'', and the discovery of its cetacean-like inner ear, simultaneously put an end to the idea that whales were descended from mesonychids, while demonstrating that ''Pakicetus'', and all other cetaceans, are artiodactyls.


See also

* Evolution of cetaceans


Notes


References

* * * * * * * * * * * {{Taxonbar, from=Q310503 Transitional fossils Pakicetidae Extinct animals of Pakistan Eocene mammals of Asia Prehistoric cetacean genera Fossil taxa described in 1981