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Early giant whale from Australia changes our understanding of whale evolution
A jawbone fragment overlooked for more than 100 years was once part of a nine-metre-long ancient whale.
While the fossil hasn’t been described as a new species, it suggests that the huge size of modern baleen whales may have their origins south of the equator.
A fossil from a ‘dark age’ of palaeontology reveals that giant whales evolved much earlier than expected.
Discovered on the bank of Australia’s Murray River, the fragment of jawbone may not be much to look at. This might explain why the bone sat in the collections of Museums Victoria for over a century.
But a new study of the fossil reveals that the ancient relatives of the blue whale were already reaching giant sizes by 20 million years ago, measuring as much as nine metres long. It suggests that giant baleen whales have their origins in the southern hemisphere, something researchers previously hadn’t recognised.
Dr James Rule, a researcher at the Natural History Museum and lead author of the paper, says, ‘Though nine metres isn’t large for a whale today, this would have been around 1.5 times as big as the other baleen whales we knew were living at the time.’
‘This fossil reveals that whales in the southern hemisphere were getting larger earlier than expected, upending studies based mostly on fossils from north of the equator.’
Co-author Dr Erich Fitzgerald, from Museums Victoria, adds, ‘The southern hemisphere, and Australia in particular, have always been overlooked frontiers for fossil whale discovery.’
‘Fossil whale finds, like the specimen from the Murray River, are shaking up the evolution of whales into a more accurate, truly global picture of what was going on in the oceans long ago.’
The paper was published in the journal Proceedings of the Royal Society B.
Researchers used the size of the jaw fragments to estimate how large the whale would have been overall. Image © Eugene Hyland/Museums Victoria.
The whale ‘dark ages’
Baleen whales are the largest animals ever to have lived. Using plates made out of keratin to filter fish and invertebrates out of the water, the marine mammals have been able to support enormous body sizes for millions of years.
Finding out when they first became gigantic, however, has proven difficult. Many cetacean fossils from the early Miocene which might reveal this transition haven’t survived.
‘During the early Miocene between 23 and 16 million years ago, there is a gap in the fossil record of cetaceans,’ James says. ‘At this time, the sea level is thought to have fluctuated, removing many of the shallow marine environments whales and other marine mammals relied on.’
‘This means that, not only are there less sites where these fossils might be preserved, but the whales are less likely to have been in the shallows at all.’
Combined with a lack of exploration for these fossils in the southern hemisphere, this means only 20% of the world’s baleen whale fossils come from below the equator. This means any marine mammal fossil from the southern hemisphere can offer an important insight into a little-known part of history.
The fossil was found along the riverbank of the Murray river near Wongulla. Image © Steven Giles/Shutterstock.
Solving the marine mammal mystery
It turns out that the evidence researchers were looking for had already been found and was sitting in the collections of Museums Victoria. Discovered in 1921 near Wongulla, South Australia, the fossil had been largely unappreciated for what it was.
Part of the reason is because Wongulla is more than 100 kilometres inland, hardly the first place anyone would go looking for whale fossils. Even 19 million years ago when the whale was alive, this would have been a long way from the open ocean, suggesting that the animal may have arrived there by accident.
‘In the past, this area would have been a shallow seaway, which would have been a rare environment for this kind of animal,’ James explains. ‘It seems like it may have entered the seaway and become stranded, with the fossil surviving until the modern day.’
Erich only realised the fossil’s significance around a decade ago, when he gave it to James to work on for his undergraduate project. Using other whale fossils, they were able to estimate that the Murray River fossil was the largest living baleen whale at the time.
While there’s not enough evidence to describe the jaw fragment as a new species in its own right, the fossil has provided important new data for ancient whale researchers.
When used to compare the size of baleen whales over time, it showed that whales from the north only shifted to larger sizes within the past five million years, whereas those in the south have been larger for much longer.
Following on from other discoveries like Perucetus earlier this year, the team now hope to find more evidence of large whales in the southern hemisphere. This will help to resolve ongoing questions about these mighty mammals, and help reveal just why they became so large.
‘This discovery goes to show that Australia and New Zealand are an emerging area of fossil whale research,’ James says. ‘Another big whale is almost certain to turn up at some point, and we hope this research will inspire people to track them down.’
