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Paleocene period - Mesonychids were wolf-like animals, with 4 hooves/paws, used to walk on land.
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Eocene period - The Pakicetus was a four-legged, land mammal, resembling the crocodile. The Pakicetus had a thickened skull bone, known as the auditory bella, which is used for underwater hearing.
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Early Eocene Period - The Ambulocetus was an amphibian, crocodile-like animal. This animal had four legs, which were better suited for swimming rather than walking. This animal also had an inner ear bone, which enabled it to hear underwater.
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Late Early Eocene Period - The Dalanistes was also a four-legged, with long narrow skulls, and an openings at the end of the skull.
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Lutetian Period - The Rodhocetus was an early whale that had four, short legs, with webbed hands and feet.
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Narwhals are medium-sized, slim whales that live in the Arctic Circle. Their distinguishing trait is the horn/tusk that comes from the upper lip of male Narwhals. The Narwhals do not have any pelvis-like structures to show they did, at one point have four legs.