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 with short limbs and large feet used for swimming and perhaps also walking on land. It lacked the tail fluke, present in later early whales. Its pelvis was attached to its spine, like land mammals and unlike later whales.[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; but recent studies show that it was fully aquatic like modern cetaceans.[2]

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).[3] When the animal was alive, Pakistan was a coastal region of India, which was then an island continent in the Tethys Ocean.

Description[edit]

Life reconstruction of Ambulocetus natans

Fewer than ten ambulocetid fossils have been found, all 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 on each hand and its long hindlimbs had four toes on 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. Its skull had a long snout and eyes facing sideways (whereas they faced upward in pakicetids) located high on the skull like in modern hippos.[4]

Thewissen, Madar & Hussain 1996 estimated the body weight of Ambulocetus to 141–235 kg (311–518 lb),[5] though Philip D. Gingerich estimated it to 720 kg (1,590 lb).[6]

Like modern cetaceans it was fully aquatic,[7] with a similar thoracic morphology, and like them it probably swam by undulating its back vertically. Chemical analysis of its teeth shows that it could move between salt and fresh water. Ambulocetus did not have external ears.

Pakicetids and ambulocetids used their large feet and hind limbs for propulsion. Morphologically, the thigh and leg were shortened but the feet stayed 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.[8]

Ambulocetus's was 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 thrashing it with violent motions. Similar to larger crocodilians, adult Ambulocetus probably were ambush predators that fed on larger fishes, aquatic tetrapods and possibly terrestrial animals near the water. In contrast to crocodilians, it may have chewed its prey but probably did minimal food processing with its teeth.[9]

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 bone's structure was like those of whales, enabling it to hear well underwater. In addition, its teeth are similar to those of early cetaceans.[10]

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.[11] It is believed to be from the Lutetian age of the Paleogene period (48.6 to 40.4 million years ago).[12]

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.[13] Together with Basilosauridae, the five families are classified under the suborder Archaeoceti.[14]

See also[edit]

References[edit]

Notes[edit]

  1. ^ Kemp 2005, p. 265
  2. ^ Konami Ando, Shin-ichi Fujiwara, Farewell to life on land – thoracic strength as a new indicator to determine paleoecology in secondary aquatic mammals, First published: 10 July 2016 DOI: 10.1111/joa.12518
  3. ^ "H-GSP 9210 (Eocene of Pakistan)". Paleobiology Database. 2005. Retrieved June 2016.  Check date values in: |access-date= (help)
  4. ^ Thewissen et al. 2009, Ambulocetidae: the First Marine Cetaceans, pp. 279–280
  5. ^ Thewissen, Madar & Hussain 1996, Body shape and weight, p. 69
  6. ^ Thewissen 2013, pp. 457–458
  7. ^ Konami Ando, Shin-ichi Fujiwara, Farewell to life on land – thoracic strength as a new indicator to determine paleoecology in secondary aquatic mammals, First published: 10 July 2016 DOI: 10.1111/joa.12518
  8. ^ Thewissen et al. 2006, pp. 8414, 8415–8416
  9. ^ Thewissen, Madar & Hussain 1996, Modern Analogues, p. 62
  10. ^ Thewissen, Madar & Hussain 1996
  11. ^ Thewissen, Hussain & Arif 1994
  12. ^ Ambulocetus in the Paleobiology Database. Retrieved September, 2013
  13. ^ Uhen 1998, p. 43
  14. ^ Rose 2006, p. 273

Sources[edit]

External links[edit]

  • "Ambulocetus". BBC Nature. Retrieved June 2016.  Check date values in: |access-date= (help)
  • "Whale Origins". Thewissen Laboratory. Retrieved May 2016.  Check date values in: |access-date= (help)