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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, a characteristic shared only with a single modern-day crocodile hailing from Australia.
How do paleontologists know that Ambulocetus was ancestral to whales? For one thing, the bones in this creature's ears were similar to those of modern cetaceans, as was its ability to swallow underwater and its whale-like teeth. That, plus its similarity to other known whale ancestors like Pakicetus and Protocetus, pretty much seals the cetacean deal.