Temporal range: Early Eocene, 47.8–41.3 Ma
Thewissen, Hussain & Arif 1994
Thewissen, Hussain & Arif 1994
Ambulocetus was an amphibious, early cetacean that had reduced limbs that could support it on land and large feet used for paddling in water. It lacked the tail fluke, present in later early whales, and had a pelvis attached to the spine, in contrast to later whales, and, hence, it could walk as well as swim. Along with other members of Ambulocetidae, it is a transitional fossil that shows how whales evolved from land-living mammals. It is also named the walking whale because of this.
Ambulocetus was recovered from the Early Eocene (47.8-41.3 Ma) Kuldana Formation of Punjab, Pakistan ( , paleocoordinates ). When the animal was alive, Pakistan was a coastal region of India, which was then an island continent in the Tethys Ocean.
Fewer than ten ambulocetid fossils have been found, all of which have been found in shallow sea or coastal swamp environments. A. natans is the only virtually complete skeleton known. Ambulocetus was the size of a male sea lion, much larger than Pakicetus. Its short forelimbs had five fingers in each hand and and its long hindlimbs had four toes in each foot. It had dense osteosclerotic limb bones, suggesting it was well-adapted for living in water but moved slowly, probably hunting as an ambush predator. Its powerful tail was apparently used for locomotion and it probably moved similar to a modern river otter. It has a skull with long snout and eyes facing sideways (whereas they were facing upward in pakicetids) located high on the skull like in modern hippos.
It was clearly amphibious, as its back legs were better adapted for swimming than for walking on land, and it probably swam by undulating its back vertically, as otters and whales do. It has been speculated that Ambulocetids hunted like crocodiles, lurking in the shallows to snatch unsuspecting prey. Chemical analysis of its teeth shows that it could move between salt and fresh water. Ambulocetus did not have external ears. To detect prey on land, they may have lowered their heads to the ground and felt for vibrations.
Pakicetids and ambulocetids used their large feet and hind limbs for propulsion. Morphologically, the thigh and leg were reduced in length but the feet remained large; this resulted in a reduction in lever arm but a retention of a large propulsive surface — the hind limb functioned as an oar. In later Eocene cetaceans, basilosaurids and remingtonocetids, the tail has osteological evidence for a fluke and is the dominating propulsive tool while the leg is reduced and rudimentary.
Ambulocetus had a feeding morphology similar to that of crocodiles: a long snout, pointed teeth, and strong jaw adductor muscles. Like crocodilians, Ambulocetus probably killed its prey by holding it in its jaw and either drowning it or trashing it using violent motions. Similar to larger crocodilians, adult Ambulocetus probably were ambush predators that fed on larger fishes in shallow water and mammals near shores. In contrast to crocodilians, it may have masticated its prey but probably did minimal food processing with its teeth.
Scientists consider Ambulocetus to be an early whale because it shares underwater adaptations with them: it had an adaptation in the nose that enabled it to swallow underwater, and its periotic bones had a structure like those of whales, enabling it to hear well underwater. In addition, its teeth are similar to those of early cetaceans.
Ambulocetus was recovered from the Upper Kuldana Formation of Pakistan in 1993 by Johannes G.M. Thewissen and Sayed Taseer Hussain. It was described by Thewissen, Hussain, and Mohammad Arif in 1994. It is believed to be from the Lutetian age of the Paleogene period ( ).
Ambulocetus is classified under the monophyletic family Ambulocetidae. The family is believed to have diverged from the more terrestrial Pakicetidae. The families Protocetidae and possibly Remingtonocetidae, are believed to have arisen from a common ancestor with ambulocetids. Together with Basilosauridae, the five families are classified under the suborder Archaeoceti.
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