| Pakicetidae | |
|---|---|
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| Pakicetus | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | Artiodactyla |
| Infraorder: | Cetacea |
| Informal group: | †Archaeoceti |
| Family: | †Pakicetidae Thewissen, Madar & Hussain 1996 |
| Genera | |
Pakicetidae ("Pakistani whales") is anextinctfamily ofArchaeoceti (early whales) that lived during theEarly Eocene inPakistan.[1] Unlike modern cetaceans, they had well-developed limbs and were capable of walking. The species included were fox to wolf-sized.[2]
Dehm & Oettingen-Spielberg 1958 described the first pakicetid,Ichthyolestes, but at the time they did not recognize it as a cetacean, identifying it, instead, it as afish-eatingmesonychid. Robert West was the first to identify pakicetids as cetaceans in 1980 and, after discovering a braincase, Phillip Gingerich and Donald Russell described the genusPakicetus in 1981. 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 completeskeleton from a single individual has been recovered.[3]The pakicetid goldmine is the "H-GSP Locality 62" site in theKala 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.[2]
Pakicetids have been found in or nearriver deposits in northern Pakistan and northwesternIndia, a region which was probablyarid with only temporary streams when these animals lived there. No pakicetids have been found in marine deposits, and they were apparentlyterrestrial 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 asballast; they clearly indicate pakicetids were not fast runners notwithstanding their otherwisecursorial morphology. Most likely, pakicetids lived in or near bodies of freshwater and their diet could have included both land animals and aquatic organisms. During theEocene,Pakistan was an island-continent off the coastal region of theEurasian land mass and therefore an ideal habitat for the evolution and diversification of the Pakicetids.[3]
| Pakicetids have manyapomorphic traits (derived traits shared by several taxa) found inartiodactyls, including:[1] | Traits linking pakicetids to cetaceans include:[1] |
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Pakicetid ears had anexternal 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, themandibular 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. Thetympanic bulla in pakicetid ears is similar to those in all cetaceans, with a relatively thin lateral wall and thickened medial part known as theinvolucrum. However, in contrast to later cetaceans, thetympanic bone makes contact with theperiotic 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.[4]
Interpretations of pakicetid habitat andlocomotory behaviour varies considerably:
In 2001, it was concluded by Thiwissen et al. that "pakicetids were terrestrial mammals, no more amphibious than atapir." According to them, none of theaquatic adaptations found in the oldest obligate aquatic cetaceans,basilosaurids anddorudontids, 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 therevolutezygapophyses (processes between the vertebrae) like in stiff-backed runners such asmesonychians. The sacral vertebrae are fused and the sacroiliac joints present like in land mammals and amphibious cetaceans.[5]
Furthermore, according toThewissen et al., the pakicetidscapulae have largesupraspinous fossae with small acromions, in contrast to other cetaceans. Thedeltopectoral crests 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.[5]
Gingerich 2003disagreed and got support fromMadar 2007:postcranial 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.[1] This view was further reiterated by Gingerich in a 2017 paper.[6]
