Whales weren't always the giants of the sea that we know today – their ancestors plodded around on land before taking to a more aquatic lifestyle. Now a team of palaeontologists has uncovered the fossil bones of a strange new "missing link" whale species, which had four legs and was amphibious.

Named Peregocetus pacificus, the whale species was dated to about 42.6 million years ago, during the middle Eocene epoch. Measuring 4 m (13 ft) long, its four legs, tail and snout make it look more like an otter than a whale.

The creature was clearly just as comfortable on dry land as it was in the water. The team discovered tiny hooves on the tips of its toes – a holdover from its land-dwelling ancestors, which also gave rise to animals like camels and pigs. But they also found clues that indicated those toes were webbed, helping Peregocetus swim. The structures of the vertebrae in its tail, similar to those in beavers and otters, also suggested that its tail played a key role in swimming.

Although Peregocetus isn't the first four-limbed amphibious whale species to be discovered, it does help plug a big hole in the origin story of whales. It's long been thought that these amphibious whales originated somewhere around southern Asia more than 50 million years ago, before making their way westward to Africa, then the Americas.

This discovery fits into that narrative nicely. These bones were found in southern Peru, in a coastal desert region named Playa Media Luna, indicating they'd reached South America 42.6 million years ago. After that, it's believed they migrated up to North America.

"This is the first indisputable record of a quadrupedal whale skeleton for the whole Pacific Ocean, probably the oldest for the Americas, and the most complete outside India and Pakistan," says Olivier Lambert, corresponding author of the study. "We will keep searching in localities with layers as ancient, and even more ancient, than the ones of Playa Media Luna, so older amphibious cetaceans may be discovered in the future."

The research was published in the journal Current Biology.

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