Science

Fossils suggest how whales found their swimming style


It sounds like a Rudyard Kipling story but fossil-hunters say they have new clues as to how the whale came to move.

Whales as we know them today evolved over millions of years from terrestrial creatures to semi-aquatic animals to fully aquatic species, with forelimbs becoming flippers, the fluked tail developing and well-developed hind legs – once used for swimming – lost over time.

Now experts say fossils of a previously unknown early whale are offering fresh hints as to how modern whales ended up propelling themselves.

“How do you go from a foot-powered swimmer to a tail-powered swimmer? That is the kind of intermediacy we have been looking for,” said Philip Gingerich, a co-author of the study and professor emeritus at the University of Michigan. “The new whale is too late to be in the direct line [to modern whales], but it gives us a sense of how the transition may have taken place.”

Writing in the journal Plos One, the team reports that both a partial and an almost complete fossil of the new creature were unearthed in Egypt in 2007 at a site known as Wadi al-Hitan or “Valley of Whales” – a nod to the large variety of other beasts that have previously been discovered there.

The new species, called Aegicetus gehennae, was about three and a half metres long and is likely to have lived about 35m years ago, towards the end of a period known as the Eocene, and is the youngest known member of group of four-limbed, semi-aquatic animals, known as a protocetids, that were splashing about from around 49m years ago.

The word Aegicetus refers to both the creature’s breast bone, which resembles a shield, and the Latin for whale, while gehennae, or “hell”, refers to the area in which it was found.

While most protocetids are thought to have propelled themselves in water using their feet, the new species shows features that suggest it might also have moved by undulating its body in the water, and was likely fully aquatic.

That shift in propulsion, the team say, is important as it could offer insights into how the movement of whale-like animals changed over millennia.

Among the telltale features of the new species, the team say the hind limbs are slightly smaller than in other protocetids, while both the hind limbs and pelvis unusually lack a direct connection to the backbone.

“This doesn’t have tiny hind limbs, it still has pretty good-sized ones,” said Gingerich. “[But] they are a little smaller than the hands, which is unusual for protocetids – usually the feet are quite a bit bigger because they swim with their feet, they push water with their feet,” he said.

Meanwhile the animal’s backbone is larger and more elongated than those of other protocetids, giving the creature more flexibility that could have allowed it to wiggle its body to help it move.

The new discovery does not directly answer the question of how modern whales came to be: the animals that eventually gave rise to modern whales branched off from protocetids long before Aegicetus gehennae swam about.

But Gingerich said it did suggest similar evolutionary changes might have occurred during that branching process, changes that led to ancestors of modern whales such as basilosaurids. These creatures had big teeth but tiny hind limbs and moved through the water in an eel-like fashion by undulating their bodies.

It seems the newly discovered creature would have been swimming the seas at the same time.

“I think basilosaurids are swimming with their bodies – not with their tails per se, because they don’t have much of a fluke – and what we see in this new protocetid is that it looks like it was swimming more like basilosaurids,” he said.



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