In Depth
Pakicetus may well be the oldest known ancestor to modern whales, and along with other animals such as Ambulocetus, represent the group of terrestrial carnivores that began developing adaptations for a wholly aquatic lifestyle.
Due to its transitional nature, it is hard to say with certainty how Pakicetus lived. Its ear structure is more developed for hearing in the air. Other forms such as the aforementioned Ambulocetus had ears that worked best when submerged in the water. This indicates that Pakicetus itself spent more time out of the water than in it.
Analysis of the fossil site indicates that it was a coastal region at the time, and as such possibly had many estuaries and islands. Pakicetus has been envisioned by some as a wolf sized predator that would dive into the water after fish. In this scenario it could have focused its attention upon prey that had become trapped in tidal pools at low tide.
The bones of Pakicetus indicate dense bone growth, a well-known adaptation in animals that spend a lot of time in the water. This bone analysis combined with the positioning of the eyes near the top of the skull has brought the second hunting theory of lurking in the water like a crocodile.
Either way it may be that these marine adaptations are not just driven by predatory forces but practical ones too. Pakicetus may have had to cover a lot of ground to find food, and the rising and falling tides may have cut off some areas of the coast, creating islands at high tide that would have had passable land bridges at low tide. By evolving into a body that could provide efficient aquatic movement, Pakicetus would have had an advantage in not having to wait for the low tide.
Further Reading
– Middle Eocene large mammal assemblage with Tethyan affinities, Ganda Kas region, Pakistan. – Journal of Paleontology 54(3):508-533 – R. M. West – 1980. – Pakicetus inachus, A New Archaeocete (Mammalia, Cetacea) from the Early-Middle Eocene Kuldana Formation of Kohat (Pakistan) – Contributions from the Museum of Paleontology, The Museum of Michigan 25 (11): 235–246 – Philip D. Gingerich & Donald E. Russell – 1981. – Origin Of Underwater Hearing In Whales – Nature 361 (6411): 444–445. – J. G. M. Thewissen & S. T. Hussain – 1993. – New middle Eocene archaeocetes (Cetacea: Mammalia) from the Kuldana Formation of northern Pakistan. – Journal of Vertebrate Paleontology 29(4):1289-1299 – L. N. Cooper, J. G. M. Thewissen & S. T. Hussain – 2009. – From Land to Water: the Origin of Whales, Dolphins, and Porpoises – Evolution: Education and Outreach 2 (2): 272–288. – J. G. M. Thewissen, L. N. Cooper, J. C. George & S. Bajpai – 2009.
