Bat poo may have preserved the world's oldest sperm, which has been found at a fossil site in Australia, say researchers.

The discovery of giant sperm along with other internal reproductive organs in exceptionally well-preserved 17-million year old fossil "seed shrimp" or ostracods is reported this week in the Proceedings of the Royal Society B.

"Nobody has ever seen sperm fossilized like this before," says co-author palaeontologist Professor Mike Archer from the University of New South Wales, in Sydney. "This is the oldest sperm in the world."

"We get used to fossil bones and teeth but we did not expect the soft tissues would also preserve for 17 million years."

Ostracods are tiny crustaceans that have been around for 500 million years.

Fossil ostracod sperm

The giant fossil sperm were coiled and up to a centimetre long - about 50,000 times bigger than a human sperm. ( Renate Matzke-Karasz )

"They're cute little guys. They have two little shells and if you look inside, it looks like a little crab tucked up inside desperate to avoid being seen by the rest of the world," says Archer.

He and colleagues found male and female ostracods fossilised in deposits at the Bitesantennary Site in the Riversleigh World Heritage area of the Australian state of Queensland.

Archer says the site was once a pool inside a rainforest-shrouded cave that was home to numerous bats.

"There were zillions of bat bones so we know there were bats," he says.

Archer says as the ancient ostracods were going about their daily business in the pool when something went wrong.

"What appears to have gone wrong is a torrential rain of bat droppings."

Archer says the leaf-nosed bats, related to those seen in fossil deposits in Europe, would have converted the insect life of the rainforest into bat poo.

"And when they got back into the cave, they let it drop."

He says this conclusion is supported by a chemical analysis of the rock from the cave which shows it is filled with phosphorus.

Archer says the simplest explanation for the find is that the pool was periodically so loaded with phosphorous that it fossilised anything that fell in it.

"And it happened so fast that soft tissues apparently were preserved," he says.

Associate Professor Suzanne Hand says the bats' likely role in the preservation of ostracod sperm cells is doubly interesting.

"This amazing discovery is echoed by a few samples of exceptional soft-tissue preservation in fossil bat-rich deposits in France, hence the key to eternal preservation of soft tissues may indeed be some magic ingredient in bat droppings."

Caught in the act

Using synchrotron x-ray tomography and a new high resolution nanotomography procedure, team members in Europe got a full view of the ostracods' internal reproductive organs.

Ostracod

Ostracods are crustaceans (a group that includes shrimp and crabs) that have been around for 500 million years. Above is a modern freshwater ostracod that uses giant sperm for reproduction, like the fossil ostracod. (Renate Matzke-Karasz)

"Suddenly every aspect of the ostracod could be seen," says Archer.

While ostracods are just a few millimetres long, their elaborate reproductive system accounts for one-third of the mature adult's volume.

The males are famous for having some of the longest sperm in the animal kingdom — around one centimetre in length.

The researchers captured images of this bizarre sperm coiled inside the male, and also saw the sperm pumps that transfer it to the female via seminal ducts.

And in the female ostracods the researchers saw sperm trapped in the large seminal receptacles that store sperm before fertilisation.

"These guys were actively in a state of reproduction. They were going about it and then suddenly tables turned and all of a sudden all biological activity stopped and they were sent into the future as a time capsule just doing what they were doing," says Archer.

Cells' insides preserved too

Not only were reproductive organs preserved, but so were the structures within cells.

The researchers saw coiled nuclei in the sperm that once contained chromosomes.

"We've never seen the intracellular structures preserved before. It's the nature of this preservation that has boggled everybody's mind," says Archer.

The preserved sperm look no different to those found in modern ostracods, he says.

"This suggests that their mode of reproduction represents a functionally successful model," says co-author Dr Renate Matzke-Karasz of Ludwig-Maximillian-University and Geo-Bio-Center in Munich.

Matzke-Karasz helped analyse the fossils sent to Europe by Australian ostracod expert John Neil from La Trobe University.

Apart from the ostracod soft tissues, Riversleigh has also yielded fossils of 19- to 20-million-year-old uncrushed insects including butterflies, leaves and even eyeballs within the skulls of extinct marsupials, says Archer.