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From land to sea: Whale evolution and adaptation

by Janice Wormworth

 

Hans Thewissen examining the pelvis of Ambulocetus natans, the 49-million-year old ‘walking, swimming whale’ discovered in Pakistan by Thewissen and his team in 1992. Courtesy Hans Thewissen. Reproduced with permission.

Palaeontologist Hans Thewissen should have been content with his new discovery: the fossil of an ancient four-legged animal. But he wasn’t.

‘I was so frustrated…I had this nice complete skeleton in three wooden crates, but I didn’t know what the stupid thing was.’

It was 1992, and Thewissen and his team had spent three arduous days digging the 49-million-year-old fossil out of the Pakistan desert. They took it back to the compound where they were staying, and wrapped it carefully in plaster and toilet paper, to prepare it for shipment back to Thewissen’s lab in the United States for future study. But Thewissen couldn’t wait that long.

Hans Thewissen holding the skull of a fossil whale known as Remingtonocetus. Courtesy Hans Thewissen

He began unwrapping a few pieces, starting with the ear, and immediately noticed an unusually heavy bone. This left him more baffled than ever, because it did not match an animal group related to elephants and sea cows—his best guess for the mystery fossil’s identity.

‘So then I unwrapped the lower jaw. I expected to see these low molars, like in elephants and sea cows, but they were these high teeth. Suddenly it hit me, “Oh, this is a whale”. And then I thought, “Oh my God, I understand why that ear looked like that!” It was a whale ear.’

Thewissen was shocked because he didn’t expect to find something that had never been found before—very large hind limbs on a fossil whale. ‘That was a real game changer,’ he says.

Named Ambulocetus natans, or ‘walking whale that swims’, Thewissen’s find joined a lengthening procession of remarkable, unexpected and sometimes baffling fossil cetaceans. Teased from rock over the past two and half decades, they provide one of the best-documented and most spectacular cases of evolutionary transformation.

Around 50 million years ago, whales’ mammal ancestors began their journey towards an aquatic lifestyle, reversing a process begun 200 million years earlier when animals first emerged from the seas. But evolution never truly goes backwards: whales did not develop gills and scales, but instead a whole new suite of aquatic adaptations. Other mammals have turned to the oceans—sea manatees, dugongs, sea otters, polar bears, seals, sea lions and walruses—but no other group surpasses whales in terms of diversity and number of species.

An artist's impression of Ambulocetus Natans, the 'walking whale' discovered by Thewissen and his team in Pakistan. Illustration by Roman Uchytel. Reproduced with permission.

Indohyus: Ancient link to whale origins

Before turning to whales’ ancient ancestors (called the Archaeoceti), it helps to understand a fossil that tells us about their origins and relatives. Indohyus was the size of a cat but shaped like a deer, and belonged to the order Artiodactyla, or even-toed ungulates. This large group today includes deer, hippopotamus, cows, giraffes, sheep and pigs.