Ambulocetus natans skeleton
Ambulocetidae is a family of early cetaceans from Pakistan that still were able to walk on land. The genus Ambulocetus, after which the family is named, is by far the most complete and well-known ambulocetid genus due to the excavation of an 80% complete specimen of Ambulocetus natans. The other two genera in the family, Gandakasia and Himalayacetus, are known only from teeth and mandibular fragments.
The most basal of amphibious marine cetaceans, ambulocetids lived in shallow near-shore environments such as estuaries and bays, but were still dependent on freshwater during some stage of their life. Some of the characteristics related to sound transmission found in the lower jaws of modern whales that were absent in pakecetids are present in ambulocetids. They probably swam by paddling their large feet, which is not a very efficient mode of locomotion, suggesting they ambushed rather than chased prey. Ambulocetids had a narrow head with eyes facing laterally, giving them an appearance similar of theropod dinosaurs. Ambulocetus and Gandakasia primarily ate terrestrial prey, while a combination of low oxygene isotope and high carbon isotope values suggests that Himalayacetus consumed freshwater but ate marine prey, thus that it foraged in a marine environment but returned on land to drink.
Ambulocetus natans was an early cetacean that could walk as well as swim. It is the only species classified under the genus Ambulocetus. Along with other members of Ambulocetidae, it is a transitional fossil that shows how whales evolved from land-living mammals. Ambulocetus natans lived in the Early Eocene (50 to 48 million years ago) of Pakistan. When the animal was alive, Pakistan was a coastal region of India, which was then a large island in the Indian Ocean.
Having the appearance of a 3 meter (10-foot) long mammalian crocodile, it was clearly amphibious, as its back legs are 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 was able to 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.
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 natans 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 late Ypresian to the early Lutetian ages of the Early to Middle Eocene (50 to 48 million years ago).
Gandakasia was a genus of ambulocetid from Pakistan, that lived in the Eocene epoch. It probably caught its prey near rivers or streams.
Just like Himalayacetus, Gandakasia is only known from a single jaw fragment, making comparisons to other ambulocetids difficult.
The first ambulocetid to be described, Gandakasia was not recognized as a cetacean initially, but, like other ambulocetids, they were amphibious mammals able to move both on land and in water.
Himalayacetus is an extinct genus of carnivorous aquatic mammal of the family Ambulocetidae. The holotype was found in Himachal Pradesh, India, (31.0°N 77.0°E: paleocoordinates 3.5°N 69.7°E) in what was the remnants of the ancient Tethys Ocean during the Early Eocene (Ypresian), 55.8 to 48.6 million years ago. This makes Himalayacetus the oldest archaeocete known, extending the fossil record of whales some 3.5 million years.
Himalayacetus lived in the ancient coastline of the ancient Tethys Ocean before the Indian Plate had collided with the Cimmerian coast. Just like Gandakasia, Himalayacetus is only known from a single jaw fragment, making comparisons to other Ambulocetids difficult.
Upon its discovery, Himalayacetus was described as a pakicetid because the dentary has a small mandibular canal and a dentition similar to Pakicetus. Thewissen, Williams & Hussain 2001 assigned Himalaycetus to the ambulocetids.
Genus Ambulocetus Ambulocetus natans Genus Gandakasia Gandakasia potens Genus Himalayacetus Himalayacetus subathuensis