Temporal range: Early Eocene, 52–48Ma
|Cast of P. attocki, ROM|
Gingerich & Russell 1981
Gingerich & Russell 1981
Pakicetus, which is an extinct genus of amphibious cetacean of the family Pakicetidae, was endemic to the Eocene of Pakistan. The vast majority of paleontologists regard it as the most basal whale.
Based on the skull sizes of specimens, and to a lesser extent on composite skeletons, species of Pakicetus are thought to have been 1 metre (3.3 ft) to 2 metres (6.6 ft) meters 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 situated at the tip of the snout.
Reconstructions of pakicetids that followed the discovery of composite skeletons often depicted them with fur; however, given their close relationships with hippos, they more likely 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 toothwear, Pakicetus is thought to have eaten fish and small animals. The teeth also suggest that Pakicetus had herbivorous and omnivorous ancestors.
Pakicetus was originally described as being a mesonychid, but later research reclassified it 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 then believed to be descended from mesonychids, according to Gingerich & Russell 1981. However, the redescription of the primitive, semi-aquatic 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. Thus, Pakicetus represents a transitional taxon between extinct land mammals and modern cetaceans.
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). "This peculiarity could indicate that Pakicetus could stand in water, almost totally immersed, without losing visual contact with the air."
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.
The first fossils were uncovered in Pakistan, hence their name. They were found within the Kuldana formation located in northern Pakistan and were dated as early to 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 Sea. Speculation is that many major marine banks flourished with the presence of this prehistoric whale. According to the location of fossil findings, they preferred a shallow habitat that neighbored decent-sized land. Assortments of limestone, dolomite, mudstone and other varieties of different colored sands has been predicted to be a favorable habitat for such.
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