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Fossil find solves Australian marine mystery


Gap filled: The identification of 12-million-year-old fossils of dugong ancestors gives a new view of marine ecosystems(Source: iStockphoto)

Holy sea cow An ancient mystery has been solved through the identification of a group of fossils as the ancestors of our modern-day dugong.

The identification of the 12-million-year-old fossils pushes the record of sirenia or sea cows, the group of marine mammals that includes dugongs, in the region back seven millions years.

In doing so it also reveals the sea cow has played a critical role in the structure and maintenance of our marine ecosystems, and suggests any loss of its modern-day relative could have serious long-term impact for our oceans.

Dr Erich Fitzgerald, senior curator of palaeontology at Museum Victoria, says the fossils were found in a cave in the Highlands of Papua New Guinea almost 30 years ago by Professor Rod Wells of Flinders University in Adelaide.

Fitzgerald says it was an "extraordinarily fortuitous find" that almost cost Rod Wells his life when a flash flood washed through the cave as he was collecting the specimens, with a number of fossils lost in the flooding.

Little attention had been paid to the fossils until Fitzgerald discovered casts of the vertebrae and ribs while working at the Smithsonian Institution in the United States.

At the time Fitzgerald realised they could be the oldest sea cow fossils in Australasia and contacted Wells to see if they could work on the originals.

The results of the collaboration that also includes Dr Jorge Velez-Juarbe of the Smithsonian, is published in the latest Journal of Paleontology.

Filling the gap

Fitzgerald says their work fills a gap in sirenian geographic history, as until now the oldest sea cow fossil find in Australasia was a South Australian specimen estimated to be five million years old.

"Elsewhere in Asia, sea cow fossils are found in much older rocks, so it was always a mystery as to why fossils hadn't been found in this part of the world," he says.

Fitzgerald says the rock in which the fossils were embedded is estimated to be between 12 million to 18 million years old.

"At a minimum the fossils are 12 million years old and that extends the evolutionary history of this entire group by at least seven million years," he says.

"It takes this mammal back to a time when the world was quite different and most of Papua New Guinea was not above the sea."

Fitzgerald says it had been thought dugongs were a recent arrival to the Australasian region and therefore their role in maintaining sea grass systems has been recent.

"In fact sea cows have been part of the tropical ecosystem for a long time and it suggests the delicate interaction between sea cows and sea grasses and all other animals is quite ancient and sea cows play a vital role in maintaining the health of the marine environment."

Fitzgerald says the dugong plays a similar role under water to an elephant on the savannah.

"The elephant in Africa plays a role in modifying the savannah by making lots of little pockets of habitat that smaller animals need to survive," he says.

"Sea cows play a similar role and that suggests if they are removed or become extinct it might have serious long-term implications for the ecosystem."

Fitzgerald says Australia also has the highest amount of diversity of sea grasses in the world. This was thought to have happened because of a lack of grazing by sea cows.

Instead this discovery shows "sea cows may actually play a vital role in maintaining diversity of sea grasses".
