Ambulocetus (Latin ambulare "to walk" + cetus "whale") is a genus of early amphibious cetacean[a] 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"). Ambulocetus is known primarily from a single skeleton which is about 80% complete, and is among the most completely known Eocene cetaceans, instrumental in the study of cetacean evolution and the transition from land to sea. It had four functional limbs, and was the first cetacean discovered preserving 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 probably had a long, broad, and powerful snout, and eyes near the top of the head. Because of these features, it is hypothesised to have behaved much like a crocodile, waiting near the water's surface and ambushing large mammals, using the jaws to clamp onto and drown or thrash prey. The modern cetacean ear is highly specialised for hearing underwater and detecting certain frequencies, and Ambulocetus seems to share some of these traits. It is unclear if it used its specialised ear for hearing underwater, bone conduction on land, or if the adaptations were entirely non-functional in less aquatic cetaceans. Ambulocetus had a narrow, streamlined body, and heavy, pachyostotic ribs. It is thought to have swum much like a modern river otter, alternating beats of the hind limbs while keeping the forelimbs tucked in for most of its propulsive power, as well as undulating the torso and tail. On land, Ambulocetus may have walked much like a sea lion. It may have had webbed feet and lacked a tail fluke. The Indian subcontinent during the Eocene was an island beginning its collision with Asia, which eventually led to the uplifting of the Himalayas. India had a hot climate with tropical rainforests and coastal mangroves. Ambulocetus was found in an area which was a shallow sea off the shores of a coastal swamp or forest, and it may have predominantly inhabited brackish areas such as at a river mouth. It lived alongside requiem sharks, catfish, other fish, turtles, crocodiles, the amphibious hoofed mammal Anthracobune, and the fellow cetaceans Gandakasia, Attockicetus, Nalacetus, and Pakicetus.