World’s oldest sperm found in Australian fossil

A specimen of the modern Australian ostracod Newnhamia fenestrata with the right valve removed. Photo: Renate Matzke-Karasz Professor Michael Archer and Associate Professor Sue Hand at Bitesantennary fossil site in the Riversleigh World Heritage site. Photo: Tony Walters

A sketch of a male ostracod, showing the organisation and orientation of the reproductive system. Photo: R. Smith

The world’s oldest and best preserved sperm has been discovered in 17 million-year-old fossils unearthed in far north Queensland.

The sperm of an ancient species of freshwater shrimp, which were found inside a rock taken from the Riversleigh World Heritage area, were not only hardy, but huge compared to the animal’s body size.

The not-so-little swimmers were as long as the crustacean itself, which was only a couple of millimetres in length.

Palaeontologist Michael Archer, one of the three researchers who dug up the fossils in 1988, said: ”99.9 per cent of organisms who have sperm have, relative to their body size, small sperm [because] you’ve got to tuck them away in a little jiblet somewhere in your body.”

It was ”absolutely extraordinary” that the animal’s soft-tissue had been so well preserved, he said.

”When you’re in the field picking up hard, dry rocks and smacking them with hammers the last thing you expect to find in those rocks is a piece of what was once living soft tissue.”

Using a European synchrotron to scan the ancient critters, German researcher Renate Matzke-Karasz found many of the animal’s internal organs had been fossilised, including the sperm cells coiled inside the sex organs and the muscular pumps that thrust the sperm into the female.

Inside the giant sperm they also spied the nuclei that once held the animal’s DNA.

The fossilised shrimp, known as an ostracod, was uncovered on the floor of an ancient cave. ”The site was clearly a cave but the walls and ceiling of the cave have gone,” Professor Archer, from the University of NSW, said.

Research associate at La Trobe University, John Neil, found the fossils in the rubble after vertebrate fossils had been removed from the rock. Out of an ice-cream tub-sized container full of crushed limestone the size of gravel, Mr Neil found 800 specimens. Of those, 23 had some preserved soft tissue and just five contained well-preserved fossils with soft tissue.

”That gives you an idea of the rarity of what has happened,” he said.

A taxonomist, Mr Neil said advances in technology, such as being able to scan fossils using a synchrotron, meant a new frontier was opening up for palaeontologists, who could study the fossil record in greater detail than ever before.

The discovery, outlined in the journal Proceedings of the Royal Society B on Wednesday, shed light on the evolution of shrimps, which have been around for 400 million years. Mr Neil said there was little difference between the structure and organs of the modern-day shrimp and its ancient ancestor.

”This is indicating a long period without change which gives the evolutionary biologists something to think about, which I think is very significant,” he said.

Riversleigh is known for its beautifully preserved fossils because on the death of their owner, they were petrified in water rich in the mineral calcium carbonate, which can dissolve and re-precipitate into solid limestone rock.

”The ostracods were living in a pool in a cave and for some bizarre reason the limestone has preserved the soft tissue,” said Professor Archer.

It was unclear how the cells, including their nuclei, had been fossilised so perfectly, he said.

While soft tissue fossils are extremely rare, those that have been uncovered are usually made of bacteria that, in the process of eating the dead animal’s flesh, had become fossilised themselves.

But Professor Archer suggested it was possible the chemicals inside bat droppings, found in large quantities on the cave floor, may have played a role in preserving the sperm. ”It’s kinda magic stuff.”

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