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Scientists have discovered 'enormous' fossilised sperm dating back 17 million years.

Belonging to a tiny shrimp, the sperm, when uncoiled, was longer than the crustacean's entire body.

The preserved semen were discovered at the Riversleigh World Heritage Fossil Site in Queensland, Australia, and are the oldest ever found.

World's oldest: The world's oldest fossilised sperm has been discovered coiled up inside a tint shrimp. This image is a cross-section of 17 million year old fossil ostracod sperm. The nuclei in each sperm (dark spot) are indicated by arrows

The giant sperm are thought to have been longer than the length of the creature's body but are tightly coiled up inside the sexual organs of the fossilised freshwater crustaceans, which are known as ostracods.

'These are the oldest fossilised sperm ever found in the geological record,' said Professor Mike Archer, of the University of New South Wales' School of Biological, Earth and Environmental Sciences. He has been excavating at Riversleigh for more than 35 years.

'The Riversleigh fossil deposits in remote north-western Queensland have been the site of the discovery of many extraordinary prehistoric Australian animals, such as giant, toothed platypuses and flesh-eating kangaroos. So we have become used to delightfully unexpected surprises in what turns up there.

An artist's impression of the site 17 million years ago. The cave was in the middle of a rainforest in an area that is now part of the Riversleigh World Heritage Fossil Site in Queensland, Australia. Tiny ostracods lived in a pool of water in the cave that was continually enriched by the droppings of thousands of bats

'But the discovery of fossil sperm, complete with sperm nuclei, was totally unexpected. It now makes us wonder what other types of extraordinary preservation await discovery in these deposits.'

A research team from the university collected the fossil ostracods from the Bitesantennary Site at Riversleigh in 1988.

They were sent to John Neil, a specialist ostracod researcher at La Trobe University in Melbourne who realised they contained fossilised soft tissues.

Experts including Dr Renate Matzke-Karasz from Ludwig Maximilian University of Munich, who authored the study published in the journal Proceedings of the Royal Society B, examined the specimens at the European Synchrotron Radiation Facility in Grenoble, France.

The microscopic study revealed the fossils contain the preserved internal organs of the ostracods, including their sexual organs. This is a sketch of a male ostracod, showing the organisation and orientation of the reproductive system

The creature's zenker organs - muscular pumps used to transfer the giant sperm to the female - were preserved. 'C' shows a scanning electron microscopic image of Riversleigh fossil ostracod on its left side, 'd' shows one of the creature's zenker organs and 'e' shows details of the zenker organ, including its chitinous rings marked as (cr)

The microscopic study revealed that the fossils contain the preserved internal organs of the ostracods, including their sexual organs.

Within these are the almost perfectly preserved giant sperm cells and within them, the nuclei that once contained the animals' chromosomes and DNA.

The creature's zenker organs – muscular pumps used to transfer the giant sperm to the female – were also preserved.

The researchers estimate that the fossilised sperm are about 1.3 millimetres long, which is about the same length or slightly longer than the ostracod itself.

'About 17 million years ago, Bitesantennary Site was a cave in the middle of a vast biologically diverse rainforest. Tiny ostracods thrived in a pool of water in the cave that was continually enriched by the droppings of thousands of bats,' Professor Archer explained.

'About 17 million years ago, Bitesantennary Site was a cave in the middle of a vast biologically diverse rainforest. Tiny ostracods thrived in a pool of water in the cave that was continually enriched by the droppings of thousands of bats,' Professor Archer explained. The site of the fossil cave is pictured

Professor Suzanne Hand, a specialist in extinct bats and their ecological role in Riversleigh's ancient environments at the university, believes that the bats could have played a role in the preservation of the ostracod sperm cells.

The steady rain of faeces from thousands of bats in the cave would have led to high levels of phosphorous in the water, which could have aided mineralisation of the soft tissues.

'This amazing discovery at Riversleigh is echoed by a few examples of soft-tissue preservation in fossil bat-rich deposits in France. So the key to eternal preservation of soft tissues may indeed be some magic ingredient in bat droppings,' Professor Hand said.

Riversleigh has previously revealed insects with internal muscles that have been preserved because bacteria became fossilised as they attempted to consume the soft tissues of the creatures.

Perfectly preserved cells of leaves have been found, as well as the preserved soft tissue of eyeballs in the eye sockets of some of the extinct marsupials.

The Riversleigh fossil deposits in remote north-western Queensland (marked on the map) have been the site of the discovery of many extraordinary prehistoric Australian animals, such as giant, toothed platypuses and flesh-eating kangaroos

The researchers estimate that the fossilised sperm are about 1.3 millimetres long, which is about the same length or slightly longer than the ostracod itself. A Specimen of the modern Australian ostracod Newnhamia fenestrata is pictured