“It was filled with these wonderful bat fossils,” paleontologist Michael Archer, onetime director of the Australian Museum, told The Washington Post in a phone interview. “Bats are fascinating, too, and we thought that was all the game was.”

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The game would get a whole lot more exciting. Turns out the limestone actually contained soft tissue of an ancient muscle shrimp known as an ostracod. But it wasn’t just any soft tissue. It was perfectly preserved giant sperm, aged 17 million years.

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“These are the oldest fossilised sperm ever found in the geological record,” said Archer, who co-authored a study published Wednesday in the Proceedings of the Royal Society B. “It’s staggering.”

Even more staggering is the size of the shrimp sperm. The ostracod is a very small animal — about 1 millimeter long — but, proportionally, its sperm are huge. Uncoiled, the sperm can be “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 told USA Today, calling the fossils “amazing.” “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. “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.

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Archer contemplated the matter for a moment. “It kind of makes you feel like a dirty Peeping Tom for finding them in the middle of the act.”

But that fact adds even more mystery to the discovery, scientists say. This fossil commemorates “ancient sex with gargantuan sperm,” researcher Renate Matzke-Karasz of Germany’s Ludwig-Maximilian-University told USA Today — sex that went down immediately before the ostracods were fossilized.

It’s unclear what caused the immediate fossilization. “We don’t know how the instantaneous fossilization happened.” Archer said. “But that we don’t know what happened is part of the fun.”

Archer said he also doesn’t know for sure how the soft-tissue fossil was almost perfectly preserved for 17 million years — but he has a pretty good idea: Bat poop.

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Years ago, a similar discovery involving a fossilized frog “that still looked gooey” was made in France. Archer said bat guano may have also had a hand in the preservation of that fossil.

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“It’s some process related to bat poo,” he said. “It’s that magic ingredient somewhere in it. One day, some student is going to identify it, and someone is going to put it into face cream to combat aging.”

So, to recap: The world’s oldest sperm was just discovered, and it was shot like a spit ball at a female shrimp, and it’s gigantic, and it was preserved for 17 million years in bat poop?