How whales grew to be largest animals on Earth

A 25-million-year-old fossilised whale skull nicknamed Alfred provides a missing link between the evolution of filter-feeding whales from their toothed cousins – and explains how whales became the largest animals on Earth.

Palaeontologists from Melbourne Museum and Monash University in Australia found horizontal scratches on Alfred’s teeth, which they suggest is evidence that the ancient whale sucked prey through its chompers.

The work, published in Memoirs of Museum Victoria, shows how ancient toothed whales made the transition to filter-feeding – and triggered the rise of the biggest creatures in the world.

Today’s filter-feeding whales include the blue whale. They’re called baleen whales, after the broom-like baleen that filters krill from seawater.

It’s a highly effective feeding technique – blue whale can eat more than 3.5 tonnes of krill each day – and means the animals can grow to gargantuan sizes.

But baleen whales haven’t always boasted krill-capturing brushes. Their ancestors had teeth, much like the orca does today.

How they evolved to baleen has largely been a mystery, Museums Victoria palaeontologist Erich Fitzgerald and co-author of the work says.

“One of the big questions has been; how did this feeding style, this amazing evolutionary invention, come to be?”

Now palaeontologists have an answer.

Ancient bones discovered by fossil hunters on the coast of Washington State included a skull of Alfred, a baleen whale ancestor, from a now-extinct group called aetiocids.

These whales had teeth. But when Fitzgerald and his colleagues examined Alfred’s teeth, they found tiny horizontal scratch marks on the inner surface – a signature of an unusual type of suction feeding.

Alfred, they suspect, sucked prey such as small fish into its mouth. But with the prey also came sand, and as Alfred’s tongue moved back and forth across its teeth, the rough sand scratched the enamel.

This is seen in a few animals today, such as walrus, that use the back-and-forth movement to suck in food. But it’s the first time it’s been seen in ancient whale fossils.

“Alfred represents the smoking gun,” Fitzgerald said.

“It might just be the very beginning of even more surprising and exciting discoveries about the past lives of these extraordinary animals.”

Next, the team will check out the rest of the skeleton and other fossils to try to find more pieces of this paleontological puzzle.