Researchers have discovered the world's oldest sperm in Australia.

The fossilized sperm comes from an ancient cave in Australia's Riverslight World Heritage Fossil Site, where bat guano falling into the water may have helped preserve the cells.

The sperm belonged to a tiny crustacean called a seed shrimp or ostracod, according to The Washington Post.

"These are the oldest fossilized sperm ever found in the geological record," said Archer, who co-authored a study on the finding. "It's staggering."

Researchers also discovered millions of bat fossils inside the cave, including bat teeth, skulls, and bat guano.

"It was filled with these wonderful bat fossils," paleontologist Michael Archer, onetime director of the Australian Museum, said to The Washington Post. "Bats are fascinating, too, and we thought that was all the game was."

The ostracod is a small animal, only 1 millimeter long, but its sperm are gigantic, according to the study. The sperm can "reach up to ten times the body length of its producer," according to Science Daily.

"No one knows why ostracods have giant sperm or how they originated," David Horne of Britain's Queen Mary University of London said to USA Today. "The new evidence that they have been around for millions of years only adds to the mystery."

The sperm was found inside the reproductive tract of a fossilized female shrimp, according to The Washington Post.

The sperm are believed to be at least 16-17 million years old and fossilized in rock, which makes them the oldest petrified sperm cells ever found, according to a recent study.

 "The sperm was clearly wound up in knots within this weird 'zenker' organ, balled up like a ball of string, then literally shot at and into a female and the female catches it. It's like they were playing catch," Archer said.

The previous oldest-known ostracod sperm was just a few thousand years old. A previous sperm find does slightly beat out the ostracod in age. An insect-like springtail trapped in amber some 40 million years ago had sperm inside its body.

Preservation in amber isn't the same as preservation in rock however, as it is rare that rock preserves soft tissue, according to NBC News.

Findings were reported on May 13 in the journal Proceedings of the Royal Society B: Biological Sciences.

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