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Ambulocetus dates from the era when the ancestors of modern whales were just dipping their toes into the water: this long, slender, otter-like mammal was built for an amphibious lifestyle, with webbed, padded feet and a narrow, crocodile-like snout. Oddly, an analysis of Ambulocetus' teeth shows that this prehistoric whale thrived in both fresh and salt water lakes, oceans and rivers, a characteristic shared only with a single modern-day crocodile hailing from Australia.
How do paleontologists know that the early Eocene Ambulocetus was ancestral to whales? For one thing, the bones in this mammal's ears were similar to those of modern cetaceans, as was its ability to swallow underwater (an important adaptation for a fish-eating diet) and its whale-like teeth. That, plus its similarity to other known whale ancestors like Pakicetus and Protocetus, pretty much seals the cetacean deal.