Clifftops stand tall above the majestic River Murray, and remnants of its past show that the region — along with parts of South Australia — was once covered by a shallow inland body of water known as the Eromanga Sea.

Approximately 100 million years ago, when this sea covered the area that is now known as the Riverland, baleen whales were among the creatures that roamed its shallows.

During the Cretaceous period, the Great Artesian Basin was the site of the shallow Eromanga Sea.(SuppliedTentotwo; CC BY-SA 3.0 DEED)

In 1921, an ancient fossil of these pre-historic times was found on the banks of the River Murray opposite the shack town of Wongulla.

Francis Cudmore, an amateur palaeontologist who discovered the fossil, recognised it was of some significance and sent it to Melbourne Museum.

There it collected dust and was more or less forgotten — until recently.

More than a century later the find could now rewrite much of our understanding about the origin of baleen whales and how they became so large.

A replica of one of the dinosaur finds near Eromanga.(ABC: Anna Henderson)

Whales in the Murray?

Following new testing of the fossil, research published by Museums Victoria late last year disrupted the long-held theory that baleen whales had evolved to huge sizes in the northern hemisphere.

Monash University palaeontologist James Rule said Mr Cudmore’s fossil suggested the whales swimming in what is now Australia during Early Miocene times were much larger than previously thought.

“So the first thing we wanted to do was find out how big this [ancient whale] was, and in order to do that we measured from modern baleen whales we had in collections,” Dr Rule said.

“When we estimated the body length using these measurements we found it was about nine metres long, which was unusual because the fossil was 19 million years old.

“That is too big for a whale from that time.”

Dr Rule (left) and Dr Fitzgerald with the baleen whale fossil at Museums Victoria Research Institute.(Supplied: Museums Victoria)

Dr Rule and his fellow researchers suspected that when Mr Cudmore found it in 1921 it was sticking out of the rocks on the river bank.

“He broke the very tips off and we suspect the reason is the whole mandible was going into the cliff. It would have been impossible to remove the rest,” he said.

Flinders University research assistant Craig Westell said the landscape we are familiar with in the present day is very different to how it was when whales populated the waters.

“The Murray-Darling Basin was under a shallow sea. The whales were swimming in that sea, died, and were incorporated into the limestone that has since been exposed along the modern river,” he said.

“The river, as we know it, probably formed only 600,000 years ago, well after the sea had withdrawn from the basin.”

Blue whales are the largest of the baleen whales and the largest animals known to have existed on Earth.(Supplied: Chris Dick)

Australia enters the conversation

Ngarrindjeri elder Mark Koolmartie knows many of the stories that have been passed down through culture, in a region where whales would have swum in the Eromanga Sea, millions of years ago.

Mr Koolmartie said the whale featured prominently in one of the stories.

Mark Koolmatrie says stories from the past often help to inform the present.(ABC News: Caroline Horn)

“It is called Kondoli and it is our story of how the whale [came to be],” he said.

“In some of the caves around the north there were dolphins recorded there, so I have no doubt that things did progress up the river. One of the things we know is there was a mix of salt and fresh water.”

Dr Rule said that despite the size of the fossil, its impact on the global conversation will be large.

“The baleen whale body size has been quite a hot topic. Everyone wants to find out how whales got so big, and a lot of recent research has been done within the past five to six years,” he said.

“What we’re particularly interested in is how the fossil from Australia fits in with this story because we’re almost, usually, entirely excluded.”

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