Pakicetidae ("Pakistani whales") is an extinct
family
Family (from la, familia) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its ...
of
Archaeoceti
Archaeoceti ("ancient whales"), or Zeuglodontes in older literature, is a paraphyletic group of primitive cetaceans that lived from the Early Eocene to the late Oligocene (). Representing the earliest cetacean radiation, they include the initia ...
(early whales) that lived during the
Early Eocene
In the geologic timescale the Ypresian is the oldest age or lowest stratigraphic stage of the Eocene. It spans the time between , is preceded by the Thanetian Age (part of the Paleocene) and is followed by the Eocene Lutetian Age. The Ypresian i ...
in
Pakistan
Pakistan ( ur, ), officially the Islamic Republic of Pakistan ( ur, , label=none), is a country in South Asia. It is the world's List of countries and dependencies by population, fifth-most populous country, with a population of almost 24 ...
.
Description
described the first pakicetid, ''
Ichthyolestes'', but at the time they did not recognize it as a cetacean, identifying it, instead, it as a fish-eating
mesonychid
Mesonychia ("middle claws") is an extinct taxon of small- to large-sized carnivorous ungulates related to artiodactyls. Mesonychids first appeared in the early Paleocene, went into a sharp decline at the end of the Eocene, and died out entirely ...
. Robert West was the first to identify pakicetids as cetaceans in 1980 and, after discovering a braincase, Phillip Gingerich and Donald Russell described the genus ''
Pakicetus
''Pakicetus'' is an extinct genus of amphibious cetacean of the family Pakicetidae, which was endemic to Pakistan during the Eocene, about 50 million years ago. It was a wolf-like animal, about to long, and lived in and around water where it a ...
'' in 1981. On October 26, 2016, a publication represented the idea that the emergence of cetacea in the Paleogene presents the best idea of microevolution that resulted in the phenotype of pakicetid. During the following two decades, more research resulted in additional pakicetid cranial material and by 2001 postcranial material for the family had been described. Though all parts of pakicetid postcrania are known, no complete skeleton from a single individual has been recovered.
The pakicetid goldmine is the "H-GSP Locality 62" site in the
Kala Chitta Hills where fossils from all three genera have been found. However, this site is so littered with bones that identifying bones from a single individual is impossible, and pakicetid skeletons are consequently composites of bones from several individuals.
Pakicetids have been found in or near river deposits in northern Pakistan and northwestern India, a region which was probably arid with only temporary streams when these animals lived there. No pakicetids have been found in marine deposits, and they were apparently terrestrial or freshwater animals. Their long limbs and small hands and feet also indicate they were poor swimmers. Their bones are heavy and compact and were probably used as ballast; they clearly indicate pakicetids were not fast runners notwithstanding their otherwise
cursorial
A cursorial organism is one that is adapted specifically to run. An animal can be considered cursorial if it has the ability to run fast (e.g. cheetah) or if it can keep a constant speed for a long distance (high endurance). "Cursorial" is often us ...
morphology. Most likely, pakicetids lived in or near bodies of freshwater and their diet could have included both land animals and aquatic organisms. During the
Eocene
The Eocene ( ) Epoch is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene' ...
,
Pakistan
Pakistan ( ur, ), officially the Islamic Republic of Pakistan ( ur, , label=none), is a country in South Asia. It is the world's List of countries and dependencies by population, fifth-most populous country, with a population of almost 24 ...
was a coastal region of the
Eurasia
Eurasia (, ) is the largest continental area on Earth, comprising all of Europe and Asia. Primarily in the Northern and Eastern Hemispheres, it spans from the British Isles and the Iberian Peninsula in the west to the Japanese archipelago a ...
n land mass and therefore an ideal habitat for the evolution, and diversification of the Pakicetids.
Hearing
Pakicetid ears had an
external auditory meatus
The ear canal (external acoustic meatus, external auditory meatus, EAM) is a pathway running from the outer ear to the middle ear. The adult human ear canal extends from the pinna to the eardrum and is about in length and in diameter.
Str ...
and ear ossicles (i.e.
incus
The ''incus'' (plural incudes) or anvil is a bone in the middle ear. The anvil-shaped small bone is one of three ossicles in the middle ear. The ''incus'' receives vibrations from the ''malleus'', to which it is connected laterally, and transmit ...
,
malleus
The malleus, or hammer, is a hammer-shaped small bone or ossicle of the middle ear. It connects with the incus, and is attached to the inner surface of the eardrum. The word is Latin for 'hammer' or 'mallet'. It transmits the sound vibrations fro ...
,
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
The mandibular foramen is an opening on the internal surface of the ramus of the mandible. It allows for divisions of the mandibular nerve and blood vessels to pass through.
Structure
The mandibular foramen is an opening on the internal surfa ...
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
The tympanic part of the temporal bone is a curved plate of bone lying below the squamous part of the temporal bone, in front of the mastoid process, and surrounding the external part of the ear canal.
It originates as a separate bone (tympanic ...
in pakicetid ears is similar to those in all cetaceans, with a relatively thin lateral wall and thickened medial part known as the
involucrum
An involucrum (plural involucra) is a layer of new bone growth outside existing bone.
There are two main contexts:
* In pyogenic osteomyelitis where it is a layer of living bone that has formed about dead bone. It can be identified by radiograph ...
. However, in contrast to later cetaceans, the
tympanic bone
The tympanic part of the temporal bone is a curved plate of bone lying below the squamous part of the temporal bone, in front of the mastoid process, and surrounding the external part of the ear canal.
It originates as a separate bone (tympanic ...
makes contact with the
periotic bone The periotic bone is the single bone that surrounds the inner ear of mammals. It is formed from the fusion of the prootic, epiotic, and opisthotic bones.
References
External links
* http://www.encyclopedia.com/doc/1O8-perioticbone.html
...
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.
Locomotion
![Pakicetus attocki](https://upload.wikimedia.org/wikipedia/commons/8/84/Pakicetus_attocki.jpg)
Interpretations of pakicetid habitat and locomotion behaviour varies considerably:
In 2001, it was concluded by Thiwissen et al. that "pakicetids were terrestrial mammals, no more amphibious than a tapir." According to them, none of the
aquatic adaptation
Several groups of tetrapods have undergone secondary aquatic adaptation, an evolutionary transition from being purely terrestrial to living at least part of the time in water. These animals are called "secondarily aquatic" because although their a ...
s found in the oldest obligate aquatic cetaceans,
basilosaurids
Basilosauridae is a family of extinct cetaceans. They lived during the middle to the early late Eocene and are known from all continents, including Antarctica. They were probably the first fully aquatic cetaceans.Buono M, Fordyce R.E., Marx F. ...
and
dorudontids, are present in pakicetids. Pakicetid cervical vertebrae are longer than in late Eocene whales, the thoracic vertebrae increase in size from the neck backwards, and the lumbar and caudal vertebrae are longer than in modern cetaceans (but still shorter than in some extinct cetaceans with undulating spines.) Motion in the spine of pakicetids was further reduced by the
revolute zygapophyses (processes between the vertebrae) like in stiff-backed runners such as
mesonychians. The sacral vertebrae are fused and the sacroiliac joints present like in land mammals and amphibious cetaceans.
Furthermore, according to
Thewissenet al., the pakicetid scapulae have large supraspinous fossae with small acromions, in contrast to other cetaceans. The
deltopectoral crest Deltopectoral may refer to;
* Clavipectoral triangle, also known as the deltopectoral triangle
* Deltopectoral groove
* Deltopectoral lymph nodes
One or two deltopectoral lymph nodes (or infraclavicular nodes) are found beside the cephalic vein, b ...
s are absent in the long and slender humeri like in cursorial animals but unlike other Eocene cetaceans. Pakicetid elbows are rigid hinge joints like in running mammals and the forearms are not flattened like in truly aquatic cetaceans. In the pakicetid pelvis, the innominates are large and the ischia are longer than the ilia. The pakicetid tibiae are long with a short
tibial crest. Hindlimb features that all more reminiscent of running and jumping animals than swimming animals.
disagreed and got support from :
postcrania Postcrania (postcranium, adjective: postcranial) in zoology and vertebrate paleontology is all or part of the skeleton apart from the skull. Frequently, fossil remains, e.g. of dinosaurs or other extinct tetrapods, consist of partial or isolated sk ...
l morphology and microstructural features suggest that pakicetids were adapted to an aquatic lifestyle which included bottom wading, paddling, and undulatory swimming, but probably not sustained running. Isotopic evidence indicate Pakicetids spent a considerable part of their life in freshwater and probably ate freshwater prey.
Subtaxa
* Family Pakicetidae
[. Retrieved February 24, 2013.]
** ''
Ichthyolestes'' ()
*** ''Ichtyolestes pinfoldi'' ()
** ''
Nalacetus
''Nalacetus'' is an extinct pakicetid early whale, fossils of which have been found in Lutetian red beds in Punjab, Pakistan (, paleocoordinates ).. Retrieved June 2013.. Retrieved June 2013. ''Nalacetus'' lived in a fresh water environment, w ...
'' ()
*** ''Nalacetus ratimitus'' ()
** ''
Pakicetus
''Pakicetus'' is an extinct genus of amphibious cetacean of the family Pakicetidae, which was endemic to Pakistan during the Eocene, about 50 million years ago. It was a wolf-like animal, about to long, and lived in and around water where it a ...
'' ()
*** ''Pakicetus inachus'' ()
*** ''Pakicetus attocki'' ()
*** ''Pakicetus calcis'' ()
*** ''Pakicetus chittas'' ()
See also
*
Archaeoceti
Archaeoceti ("ancient whales"), or Zeuglodontes in older literature, is a paraphyletic group of primitive cetaceans that lived from the Early Eocene to the late Oligocene (). Representing the earliest cetacean radiation, they include the initia ...
*
Evolution of cetaceans
The evolution of cetaceans is thought to have begun in the Indian subcontinent from even-toed ungulates 50 million years ago (mya) and to have proceeded over a period of at least 15 million years. Cetaceans are fully aquatic marine mammals bel ...
Notes
References
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{{Taxonbar, from=Q1190158
Eocene first appearances
Eocene extinctions
Prehistoric mammal families