Ambulocetus

Life reconstruction of Ambulocetus natans

Ambulocetus was probably fully aquatic like modern cetaceans, with a similar thoracic morphology, and it probably swam by undulating its back vertically.[1] Chemical analysis of its teeth shows that it could move between salt and fresh water.[2] It also lacked external ears.[2] Its skull had a long snout and eyes facing sideways (they faced upward in pakicetids), located high on the skull like in modern hippos.[3]

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. [4]

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 fish, 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.[4][5]

Size

Ambulocetus compared to a man.

Measuring 10 ft (3 meters) in length, Ambulocetus was the size of a male sea lion, much larger than Pakicetus. The body weight of Ambulocetus is estimated at 311–518 lb (141–235 kg),[4] while Philip D. Gingerich estimated it at 1,590 lb (720 kg).[6]

Skull at the Natural History Museum of the University of Pisa

The short forelimbs of Ambulocetus had five fingers on each hand and its long hindlimbs had four toes on each foot. It had dense osteosclerotic limb bones,[2] 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.[7] Its powerful tail, which lacked a tail fluke,[7] 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.[8]

Ambulocetus was probably an ambush predator, hunting large and vulnerable prey much like a modern crocodile. Unlike crocodiles, all fossils of Ambulocetus have been found in sediments deposited in a nearshore marine environment.[2]

The eyes were high on the skull, suggesting a keen interest in prey above the water table. It had large feet for paddling or, when pressed together, for powering by an undulating back.[9]

The location of Ambulocetus’ nose opening (the blowhole of later whales) remains a mystery. Throughout whale evolution, the opening shifts from the tip of the snout back to the forehead. The position of the nose on Ambulocetus can only be inferred based on other early cetacean fossils, since the tip of the snout was not recovered.

A partial skeleton of Ambulocetus was found in 1992 by Thewissen working with a team from the Geological Survey of Pakistan in the Kala Chitta Hills of Punjab [10]

Ambulocetus was recovered from the Early Eocene (47.8-41.3 Ma) Kuldana Formation of Punjab, Pakistan[11] (33.6°N 72.2°E / 33.6; 72.2, paleo coordinates 14.3°N 68.3°E / 14.3; 68.3)[12] in 1993 by Johannes G.M. Thewissen and Sayed Taseer Hussain, and was described by Thewissen, Hussain, and Mohammad Arif in 1994.[13] It is believed to be from the Lutetian age of the Paleogene period (48.6 to 40.4 million years ago).[14]

While it was known that cetaceans had ancestors that lived on land, this was the first time that remains of a whale were found that included limb bones strong enough to walk on land. The fossil was published in 1994 and named Ambulocetus natans [2]

Fewer than ten ambulocetid fossils have been found, all in shallow sea or coastal swamp environments. Ambulocetus is the only virtually complete skeleton known from the group. When the animal was alive, Pakistan was a coastal region of India, which was then an island continent in the Tethys Ocean.[2]

The site was further excavated in 1996, a delay caused by the security situation of the area [2]. Longer publications followed.[10] It was dubbed as the smoking gun of whale evolution by Steven Jay Gould[15]

The most complete specimen of Ambulocetus includes the skull (without the snout), the lower jaw, the vertebral column (except for some tail vertebrae), ribs, forelimb from the elbow down, and most of the hind limb. It was found in rocks that were formed in a swampy coastal environment. It has been proposed that it was an ambush hunter, similar to modern crocodiles. The eyes were high on the skull, suggesting that it was keenly interested in prey above the water table. It had large feet used in paddling, or held together, powered by the undulating back.[11][12]

The position of the nose can only be inferred since the tip of the snout was not recovered. Both whales more basal on the phylogenetic tree (Pakicetidae) and higher ones (Remingtonocetidae) have a nose opening at the tip of the snout.[2]

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.[16] Together with Basilosauridae, the five families are classified under the suborder Archaeoceti.[17]

Ambulocetus has featured in a number of museum exhibits,[18][19][20] biology textbooks,[21] and even a cold war spy novel where Ambulocetus lives in ice caves near a submarine base in the Arctic.[22]

• Evolution of cetaceans

• "Whale Origins". Thewissen Laboratory.