Pakicetus
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''Pakicetus'' (meaning 'whale from
Pakistan Pakistan, officially the Islamic Republic of Pakistan, is a country in South Asia. It is the List of countries and dependencies by population, fifth-most populous country, with a population of over 241.5 million, having the Islam by country# ...
') is an extinct
genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...
of amphibious cetacean of the
family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...
Pakicetidae, which was endemic to the
Indian Subcontinent The Indian subcontinent is a physiographic region of Asia below the Himalayas which projects into the Indian Ocean between the Bay of Bengal to the east and the Arabian Sea to the west. It is now divided between Bangladesh, India, and Pakista ...
during the
Ypresian In the geologic timescale the Ypresian is the oldest age (geology), age or lowest stage (stratigraphy), stratigraphic stage of the Eocene. It spans the time between , is preceded by the Thanetian Age (part of the Paleocene) and is followed by th ...
(early
Eocene The Eocene ( ) is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (Ma). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene'' comes ...
) period, about 50 million years ago. It was a wolf-like mammal, about long, and lived in and around water where it ate fish and other animals. The name Pakicetus comes from the fact that the first fossils of this extinct amphibious whale were discovered in
Pakistan Pakistan, officially the Islamic Republic of Pakistan, is a country in South Asia. It is the List of countries and dependencies by population, fifth-most populous country, with a population of over 241.5 million, having the Islam by country# ...
. The vast majority of paleontologists regard it as the most 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 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 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 In jawed vertebrates, the mandible (from the Latin ''mandibula'', 'for chewing'), lower jaw, or jawbone is a bone that makes up the lowerand typically more mobilecomponent of the mouth (the upper jaw being known as the maxilla). The jawbone i ...
with some teeth. Based on the detail of the teeth, the molars suggest that the animal could rip 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 other 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 Science is a systematic discipline that builds and organises knowledge in the form of testable hypotheses and predictions about the universe. Modern science is typically divided into twoor threemajor branches: the natural sciences, which stu ...
'' in 1983 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 Tapirs ( ) are large, herbivorous mammals belonging to the family Tapiridae. They are similar in shape to a Suidae, pig, with a short, prehensile nose trunk (proboscis). Tapirs inhabit jungle and forest regions of South America, South and Centr ...
." 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 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 In anatomy and osteology, a foramen (; : foramina, or foramens ; ) is an opening or enclosed gap within the dense connective tissue (bones and deep fasciae) of extant and extinct amniote animals, typically to allow passage of nerves, artery, ...
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."


History of discovery

The first
fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserve ...
, 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 Kuldana Formation west of Islamabad in northern Pakistan and were dated as early to early-middle Eocene in age. The discovery of Pakicetus played an important role in solidifying the inferences that revolved around the evolution of whales. The fossil indicated that whales swam up and down with their vertebral column, which caused their feet to move up and down like otters and their land movements were similar to sea lions; even their limbs protracted and retracted on land. In contrast, the origin of cetaceans, which includes whales, began as four-legged land animals who actively used locomotion and were great runners as a result. 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, modern day
Pakistan Pakistan, officially the Islamic Republic of Pakistan, is a country in South Asia. It is the List of countries and dependencies by population, fifth-most populous country, with a population of over 241.5 million, having the Islam by country# ...
was part of an independent island-continent off the coastal region of
Eurasia Eurasia ( , ) is a continental area on Earth, comprising all of Europe and Asia. According to some geographers, Physical geography, physiographically, Eurasia is a single supercontinent. The concept of Europe and Asia as distinct continents d ...
, and therefore an ideal habitat for the evolution and diversification of the Pakicetidae.


Classification

''Pakicetus'' was classified as an early cetacean due to characteristic features of the
inner ear The inner ear (internal ear, auris interna) is the innermost part of the vertebrate ear. In vertebrates, the inner ear is mainly responsible for sound detection and balance. In mammals, it consists of the bony labyrinth, a hollow cavity in the ...
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. 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 do not belong. In 2001, fossils of ancient whales were found that featured an ankle bone, the 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

* Indohyus *
Evolution of cetaceans The evolution of cetaceans is thought to have begun in the Indian subcontinent from even-toed ungulates (Artiodactyla) 50 million years ago (mya) and to have proceeded over a period of at least 15 million years. Cetaceans are fully aquatic mam ...


Notes


References

* * * * * * * * * * * {{Taxonbar, from=Q310503 Transitional fossils Pakicetidae Fossils of Pakistan Eocene mammals of Asia Prehistoric cetacean genera Fossil taxa described in 1981 Taxa named by Philip D. Gingerich