Ambulocetus (from Latin ambulō, meaning "to walk", and cetus, meaning "whale", and thus, "walking whale") is a genus of early semiaquatic cetacean from the Kuldana Formation in Pakistan, roughly 48 or 47 million years ago during the Early Eocene (Lutetian). It contains one species, Ambulocetus natans (Latin natans "swimming"), known solely from one near-complete skeleton. Ambulocetus is among the best-studied of Eocene cetaceans, and serves as an instrumental find in the study of cetacean evolution and their transition from land to sea, as it was the first cetacean discovered to preserve a suite of adaptations consistent with an amphibious lifestyle. Ambulocetus is classified in the group Archaeoceti—the ancient forerunners of modern cetaceans whose members span the transition from land to sea—and in the family Ambulocetidae, which includes Himalayacetus and Gandakasia (also from the Eocene of the Indian subcontinent).
Ambulocetus had a narrow, streamlined body, and a long, broad snout, with eyes positioned at the very top of its head. Because of these features, it is hypothesised to have behaved much like a crocodile, waiting near the water's surface to ambush large mammals, using its powerful jaws to clamp onto and drown or thrash prey. Additionally, its ears possessed similar traits to modern cetaceans, which are specialised for hearing and detecting certain frequencies underwater, although it is unclear if Ambulocetus also used these specialised ears for hearing underwater. They may have instead been utilised for bone conduction on land, or perhaps served no function for early cetaceans.
It is thought to have swum much like a modern river otter, tucking in its forelimbs while alternating its hind limbs for propulsion, as well as undulating the torso and tail. It may have had webbed feet, and unlike its modern relatives, lacked a tail fluke. On land, Ambulocetus may have walked much like a sea lion<!--the study claiming it was fully aquatic is far from consensus and takes too many unsafe assumptions to be discussed in the lead-->.
Ambulocetus inhabited the Indian subcontinent during the Eocene. The area had a hot climate with tropical rainforests and coastal mangroves, and Ambulocetus may have predominantly inhabited brackish areas such as river mouths. It lived alongside requiem sharks, catfish and various other fishes, turtles, crocodiles, the amphibious hoofed mammal Anthracobune, and the fellow cetaceans Gandakasia, Attockicetus, Nalacetus, and Pakicetus.
Taxonomy
Discovery
thumb|left|[[Hans Thewissen with the holotype Ambulocetus skeleton]]
In December 1991, Pakistani palaeontologist Mohammad Arif and Dutch–American palaeontologist Hans Thewissen were jointly funded by Howard University and the Geological Survey of Pakistan to recover land mammal fossils in the Kala Chitta Hills of Punjab, Pakistan. On 3 January 1992, they recovered a small, thick rib fragment. Later in the field season, while surveying the upper Kuldana Formation, Thewissen discovered a femur (thigh bone) and proximal portion of the tibia (upper portion of the shin) which clearly belonged to a mammal. An hour later, Arif discovered the rest of the skeleton, and the two began excavation the next day. At first, Thewissen speculated the fossils belonged to an anthracobunid (a large semi-aquatic mammal), until he found the teeth near the end of the field season, which were characteristically cetacean (living cetaceans are whales, dolphins, and porpoises). Thewissen, at the time, could not afford to excavate and store everything, so he took the skull with him to the United States, while Arif kept the rest in two crates which used to hold oranges. In October 1992, Thewissen presented his research of the skull to a vertebrate palaeontology convention in Toronto, Canada. The next year, American palaeontologist Philip D. Gingerich paid for the rest of the skeleton to be shipped to the United States. In 1994, the formal description of the remains was published by Thewissen, mammal palaeontologist Sayed Taseer Hussain, and Arif. They identified the remains as clearly belonging to an amphibious cetacean, and so they named it Ambulocetus natans. The genus name comes from Latin ambulare "to walk" and cetus "whale", and the species name natans "swimming". The holotype specimen, HGSP 18507, is a partial skeleton initially discovered preserving an incomplete skull (missing the snout), some elements of the vertebral column and ribs, as well as portions of the fore- and hind-limb. Other specimens initially found were HGSP 18473 (a second premolar), HGSP 18497 (a third premolar), HGSP 18472 (a tail vertebra), and HGSP 18476 (lower portion of a femur). The holotype was found in a silt and mudstone bed over a area.
Classification
thumb|upright=1.5|Reconstructed skeletons of Ambulocetus (right) and the fully aquatic [[Basilosauridae|basilosaurid Cynthiacetus (left) at the Muséum national d'histoire naturelle, Paris]]
Modern cetaceans (Neoceti) are grouped into either the parvorders Mysticeti (baleen whales) or Odontoceti (toothed whales). Neoceti are descended from the ancient Archaeoceti, whose members span the transition from terrestrial to fully aquatic. Archaeoceti are thus paraphyletic (it is a non-natural group which does not comprise both a common ancestor and all of its descendants). Ambulocetus was an archaeocete. By the time Ambulocetus was discovered, archaeocetes were classified into the families Protocetidae (which included what are now the terrestrial Pakicetidae, and the rest were amphibious), Remingtonocetidae (amphibious), Basilosauridae (aquatic), and Dorudontidae (aquatic, now a subfamily of Basilosauridae). The earliest cetaceans were thought to be the mesonychians, proposed before any firm early cetacean fossils were identified. In the original description, Ambulocetus was preliminarily placed into Protocetidae,