By Angus Dalton
The pale slab of bone jutting from a limestone cliff along the Murray River caught the eye of an amateur fossil-hunter by the name of Francis Cudmore as he fossicked for prehistoric shark teeth in the summer of 1921.
The fossil belonged to an ancestor of the migrating humpbacks that thrill Australians along the coast each year, and the largest animal that has ever lived – the blue whale.
For almost a century, the jawbone fragment was locked away. But new analysis of the bone has blown apart assumptions that giant baleen whales evolved relatively recently and revealed their ecosystem-defining force is millions of years more ancient than previously thought.
“It’s what I call the Apollo Program: Paleontology,” Dr Erich Fitzgerald, senior curator of Vertebrate Palaeontology at the Museums Victoria Research Institute, said of his efforts to uncover the secrets of whale evolution in the face of a patchy fossil record.
“We are biased, but it makes working on dinosaurs look like a cinch.”
Fitzgerald was plumbing the catacombs of the Museums Victoria collection after-hours one night when he slid open a draw and rediscovered Cudmore’s fossil, a creamy-yellow slab that “looked like a really big slice of Bega cheese”.
The baleen whale fossil at Museums Victoria Research Institute.Credit: Eugene Hyland
He instantly recognised the fossil as the tip of a baleen whale jaw. But the size and age of the bone shocked him.
“We expect to see that in rocks that are 4 million years old,” he said. “But 19 million years ago? That was pushing credibility.”
His then-student Dr James Rule, now working at Monash University and London’s Natural History Museum, pounced on the fossil.
Rule measured more than 20 whale jaws to help calculate the prehistoric whale’s size. That included a 4.5 metre-long jaw of a blue whale – the largest bone ever produced by nature – that took five people to wrangle.
The size of a modern fin whale (black shape) compared with its ancient ancestor, the Murray River baleen whale, and a human.Credit: Ruairidh Duncan/Rob French/Museums Victoria
Rule concluded the ancient whale was 9 metres long, about the size of a modern minke. Previously, palaeontologists thought whales this size only evolved 3 million years ago in the northern hemisphere.
But in their paper Giant baleen whales emerged from a cold southern cradle, Rule and Fitzgerald argue their finding proves baleen whales actually evolved in the southern hemisphere and are at least 17 million years more ancient.
“What this implies is that you could have had baleen whales being the critical influence on the structure and healthy functioning of our oceans for some 20 million years,” Fitzgerald said.
Whales act as carbon sequesters and biological nutrient pumps, diving deep to feed and releasing plumes of faeces on the surface that fertilise blooms of oxygen-producing plankton that underpin global food chains.
Even when dead, whales’ massive carcasses sink into the deep and erupt into entire ecosystems of deep-sea feasting.
“That’s actually quite a profound thing, to think that we are now in a phase of Earth’s history where that is being upset,” Fitzgerald said. “The world’s oceans intimately rely on the big baleen whales.”
Researchers Dr Erich Fitzgerald (right) and Dr James Rule (left) argue their analysis rewrites the history of the earth’s largest creatures.Credit: Eugene Hyland
The researchers say their work has just begun.
“Just as Francis Cudmore came across these amazing fossils 100 years ago, we see ourselves now as continuing that legacy, and reigniting that adventure to uncover what I think of as the biggest mysteries in the history of life,” Fitzgerald said.
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