An ambush predator with legs, on its way to becoming a whale
Ambulocetus had a long, powerful skull with crocodile-like jaws, eyes set high on the head, and strong hind legs with large paddle-like feet. Its limb bones could still bear weight on land, but its spine and hips show an animal that swam by undulating its body and kicking its hind feet — the same up-and-down motion that modern whales make with their flukes.
Its ears were already adapting to hear underwater: the lower jaw carried a bony channel that transmitted sound to the ear the way it does in living whales. Chemical signatures in its teeth show it moved between fresh and salt water — a river-mouth animal that lived at the exact frontier where mammals were leaving the land.
Most likely it hunted like a crocodile: lying still in shallow water, then lunging at whatever came to drink. Nostrils were still at the tip of the snout — the blowhole would take millions more years to migrate to the top of the head.
Why it matters
When Darwin proposed that whales descended from land mammals, critics called it absurd — there was no fossil in between. Ambulocetus is that fossil. It is shown in nearly every biology textbook as the proof that a four-legged animal can, step by step, become a whale.