Pakicetus

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See Pakicetus in Wikipedia, the free encyclopedia.

The temporally and phylogenetically oldest known whales have been recovered from the uppermost early Eocene of Pakistan (Gingerich & Russell 1981, Gingerich et al. 1983, Carroll 1988). Though some of this material was initially referred to the Mesonychia, Gingerich & Russell (1981) correctly identified new material recovered from the Kuldana Formation outside of Punjab, Pakistan, as the most primitive known whale, and appropriately, named it Pakicetus. This specimen is an exquisite transitional form documenting the derivation of whales from terrestrial animals, such as mesonychids. The skull of Pakicetus was all that was known until 2001, but much of the dentition and braincase were preserved and displayed the mixture of apomorphic and plesiomorphic characters within this taxon. The sagittal crest is lamboidal, and high, as in Upper Eocene archaeocetes. The auditory bulla is massive, and is formed exclusively of the ectotympanic, as in other whales. Nevertheless, the auditory bullae are not isolated by sinus systems in Pakicetus but rather communicate with the squamosal, basiocciptial, and paroccipital. In modern whales, the auditory bullae articulate only with the periotic. There is a marked fossa for the tensor tympanii, strongly suggesting that Pakicetus retained a functional tympanic membrane. The middle ear does not appear to be vascularized, as in other whales. The molars are distinctly similar to those of mesonychids, including the morphology hypoconids, identical to those of mesonychids.

The Pakicetus material was recovered in association with unambiguously terrestrial organisms, and thus this animal spent at least some portion of its life on land. Gingerich et al. (1983) speculated that Pakicetus was an amphibious predator, hunting in the water and returning to rest on land. The degree to which Pakicetus was capable of swimming and diving is not entirely discernible. The lack of requisite modifications of the middle ear and auditory bullae in Pakicetus strongly suggest that it was not capable of any significant diving activity.

In 2001, further fossils were discovered (Thewissen et al., 2001), making Pakicetus appear more terrestrial and less aquatic. It is now thought to have possessed goat-like legs and been primarily a running land animal.

[edit] References

1. Carrol, R. L. 1988. Vertebrate Paleontology and Evolution. W. H. Freeman & Company, New York.
2. Gingerich, P. D. & Russell, D. E. 1981. Pakicetus inachus, a new archaeocete (Mammalia, Cetacea) from the early-middle Eocene Kuldana Formation of Kohat (Pakistan). Univ. Mich. Contr. Mus. Paleont. 25: 235-246.
3. Gingerich, P. D., Wells, N. A., Russell, D. E. & Shah, S. M. L. 1983. Origin of whales in epicontinental remnant seas: New evidence from the early Eocene of Pakistan. Science 220: 403-406.
4. Thewissen, J.G.M., Williams, E.M., Roe, L.J. and Hussain, S.T., Skeletons of terrestrial cetaceans and the relationship of whales to artiodactyls, Nature 413(6853):277-281, 20 September 2001.

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