Ambulocetus
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Ambulocetus Temporal range: Early Eocene, 47.8–41.3 Ma |
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| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | Artiodactyla |
| Infraorder: | Cetacea |
| Family: | †Ambulocetidae |
| Genus: |
†Ambulocetus Thewissen et al., 1994 |
| Species: | †A. natans |
| Binomial name | |
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Ambulocetus natans Thewissen et al., 1994 |
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Ambulocetus (meaning "walking whale") was an early cetacean with short limbs and large feet used for swimming. Along with other members of Ambulocetidae, it is a transitional fossil that shows how whales evolved from land-living mammals. While its name stems from the historical hypothesis that it was capable of walking on land, more recent research suggests that it was fully aquatic like modern cetaceans.
Ambulocetus was probably fully aquatic like modern cetaceans, with a similar thoracic morphology, and it probably swam by undulating its back vertically. Chemical analysis of its teeth shows that it could move between salt and fresh water. It also lacked external ears. Its skull had a long snout and eyes facing sideways (they faced upward in pakicetids), located high on the skull like in modern hippos.
Several features shared with other basal cetaceans indicate the close affinities of Ambulocetus with these animals; 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 other early cetaceans.
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 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.
The short forelimbs of Ambulocetus had five fingers on each hand and its long hindlimbs had four toes on each foot. It had dense limb bones, suggesting it was well-adapted for living in water but moved slowly, probably hunting as an ambush predator. Its pelvis was attached to its spine, like land mammals and unlike later whales. Its powerful tail, which lacked a tail fluke, was apparently used for locomotion, and it probably moved similar to a modern river otter. Pakicetids and ambulocetids used their large feet and hind limbs for propulsion; morphologically, the thigh and leg of Ambulocetus were shortened, but the feet stayed large; this resulted in a reduction in lever arm but a retention of a large propulsive surface, indicating that the hind limb functioned as an oar. In later Eocene cetaceans, such as the basilosaurids and remingtonocetids, the tail gained a fluke and became the dominating source of propulsion, while the leg became more reduced and rudimentary.
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