Ambulocetus

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Ambulocetus
Temporal range: Early Eocene, 47.8–41.3 Ma
Ambulocetus natans.jpg
Skeleton
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Cetacea
Suborder: Archaeoceti
Family: Ambulocetidae
Subfamily: Ambulocetinae
Genus: Ambulocetus
Thewissen, Hussain & Arif 1994
Species: A. natans
Binomial name
Ambulocetus natans
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.[1] 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 (33°36′N 72°12′E / 33.6°N 72.2°E / 33.6; 72.2, paleocoordinates 14°18′N 68°18′E / 14.3°N 68.3°E / 14.3; 68.3).[2] When the animal was alive, Pakistan was a coastal region of India, which was then an island continent in the Tethys Ocean.

Description[edit]

Restoration

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.[3]

Thewissen, Madar & Hussain 1996 estimated the body weight of Ambulocetus to 141–235 kg (311–518 lb),[4] though Philip D. Gingerich estimated it to 720 kg (1,590 lb).[5]

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.[6]

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.[7]

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.[8]

Size comparison between Ambulocetus and a human.

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.[6]

Discovery[edit]

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.[9] It is believed to be from the Lutetian age of the Paleogene period (48.6 to 40.4 million years ago).[10]

Taxonomy[edit]

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.[11] Together with Basilosauridae, the five families are classified under the suborder Archaeoceti.[12]

See also[edit]

References[edit]

Notes[edit]

  1. ^ Kemp 2005, p. 265
  2. ^ "H-GSP 9210 (Eocene of Pakistan)". Paleobiology Database. 2005. Retrieved June 2016. 
  3. ^ Thewissen et al. 2009, Ambulocetidae: the First Marine Cetaceans, pp. 279–280
  4. ^ Thewissen, Madar & Hussain 1996, Body shape and weight, p. 69
  5. ^ Thewissen 2013, pp. 457-458
  6. ^ a b Thewissen, Madar & Hussain 1996
  7. ^ Thewissen et al. 2006, pp. 8414, 8415–8416
  8. ^ Thewissen, Madar & Hussain 1996, Modern Analogues, p. 62
  9. ^ Thewissen, Hussain & Arif 1994
  10. ^ Ambulocetus in the Paleobiology Database. Retrieved September, 2013
  11. ^ Uhen 1998, p. 43
  12. ^ Rose 2006, p. 273

Sources[edit]

External links[edit]