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.

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 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.

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.

[2] The Kuldana Formation is constrained to sometime during the Lutetian stage of the Early Eocene,[3] and the remains may date to 48–47 million years ago.

[4] 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.

[6]: 2–3  Based on molecular data, cetaceans are most closely allied with hippos (Whippomorpha), and they split approximately 54.9 million years ago.

This puts mesonychians as a distant relative of cetaceans rather than an ancestor, and their somewhat similar morphology was possibly a result of convergent evolution.

Upon description, Thewissen and colleagues suggested the holotype specimen may have weighed the same as a male South American sea lion — about 300 kg (660 lb) — based on the size of the vertebrae, ribs, and limbs.

[6]: 67, 69  In 1998, based on vertebral size, Gingerich estimated a body mass of 720 kg (1,590 lb), similar to modern cetaceans.

[9] Like other archaeocetes which preserve this element, the base of the skull has an undulating contour, probably related to the shape of the nasal canal (and its passage to the throat) and the narrow infraorbital region (the area below the eyes).

Like many other archaeocetes, the pterygoids, sphenoids, and palatines form a wall lining the bottom of the nasal canal, which causes the palate to extend all the way to the ear.

The mandibular symphysis of most mammals is restricted to the midline of the jaw, but extends much farther in archaeocetes; in Ambulocetus, it reaches the back end of the first premolar.

T11–T15 have accessory anapophyses which jut straight up from the top border between the centrum and the transverse processes; and in T16, these are small, originate near the pedicles, and project tailwards.

The holotype preserves a central and a tailward sternum bone which are both exceedingly thick, about 27 mm (1.1 in) on the outer margins and decreasing towards the centre.

[5] The eight preserved lumbar vertebrae at the lower back are much longer than the thoracic, and the centra and transverse processes, from L1–L7, continually increase in length and height.

[2] The ilium of the hip of Ambulocetus, like remingtonocetids, features deep depressions to support the rectus femoris and the gluteal muscles.

Unlike terrestrial mammals and protocetids, the ischium is expanded dorsolaterally (from left to right, and upwards), which would have increased lever arm for thigh and leg retractor muscles when extended, such as while swimming.

[2] The femoral condyles of Ambulocetus are quite long compared to those of other archaeocetes and mesonychians, suggesting the knee was capable of hyperflexion (bending).

Pieces of prey are subsequently torn off by forceful, thrashing head and body motions, the feet anchoring the crocodile in place.

Since Ambulocetus was found in marine deposits (where animals would not come to drink), it is possible it hunted in river deltas which were recorded in the Kuldana Formation.

So, using river otters as a model, Ambulocetus was a pelvic paddler—swimming with alternating beats of the hindlimbs (without engaging the forelimbs)—and also undulated (moved up and down) its tail while swimming.

At higher speeds fully submerged, undulation of the spine would have become more prominent, though the feet still would have acted as the primary propulsion mechanism.

They then concluded Ambulocetus could not walk on land, but cautioned the study was limited by a lack of information on the exact density of the bone, the location of the centre of mass, and the reliance of false ribs for thoracic support.

The ectotympanic of all cetaceans, including Pakicetus and Ambulocetus, has a bony growth (involucrum) on the medial lip speculated to aid in the detection of low-frequency sounds.

These early archaeocetes may have developed such an external ear to either: better hear underwater; facilitate bone conduction of vibrations on dry land as some low-lying terrestrial creatures do (namely turtles and subterranean mole rats); or it was non-functional, and the malleus and jawbone (which are connected in the embryo stage of mammals) happened to stop separating.

[6]: 2–5  Near Locality 9209, the formation begins with 10 m (33 ft) of grey and green mud, silt, and sandstone, containing two bivalve beds.

Other localities of the upper level of the formation have yielded remains of requiem sharks, the fish Stephanodus, catfish, turtles, crocodiles,[6]: 9  and the anthracobunid Anthracobune pinfoldi.