An experimental new type of male contraceptive that blocks sperm flow, has been successful in monkey trials, thus bringing the prospect of an alternative form of birth control for humans closer. In this photo, experts believe that men should freeze their sperm as they turn 18 since the fertility rate starts declining in males as well. Renate Matzke-Karasz

A fossil holding the world’s oldest sperm was recently discovered in an ancient bat cave in Australia, and its size is impressive.

The fossilized genetic material is at least 17 million years old and is many times longer than the organism it came from. The oldest sperm, complete with sperm nuclei, belongs to an ostracod, an ancient freshwater shrimp, researchers announced on Wednesday in a study published in the Proceedings of the Royal Society B.

Like their modern descendants, prehistoric ostracods were tiny, measuring only about one millimeter in length. Their sperm, however, were relatively massive and could be ten times longer than the ostracod itself.

The ancient sperm was discovered inside four fossilized female ostracods and coiled in the seminal vesicles of one male ostracod. The prehistoric crustaceans were preserved among the millions of bat fossils at the Riversleigh World Heritage Fossil Site in northern Queensland, Australia. Paleontologists uncovered the cave in 1988.

“These are the oldest fossilized sperm ever found in the geological record,” Michael Archer, a professor at the University of New South Wales’ School of Biological, Earth and Environmental Sciences and a coauthor of the study, said in a statement. “It’s staggering.”

How did the world’s oldest sperm manage to survive millions of years of wear and tear? Researchers say that bat guano, which contains chemicals that aid preservation, kept the ancient shrimps’ anatomy intact.

Millions of years ago, the Riversleigh World Heritage Fossil Site sat in the middle of a vast rainforest. Droppings from thousands of bats provided tiny ostracods that lived in a pool of water in the cave with a continuous supply of nutrients.

Archer said the discovery of the oldest sperm was “totally unexpected.”

“It now makes us wonder what other types of extraordinary preservation await discovery in these deposits,” he said.

Scientists have previously found sperm packets from insects preserved in amber, and in insects that were frozen in the act of mating, that date back to 130 million years ago. But the ostracod fossils from Queensland are the oldest fully intact sperm that have been discovered.

There are some 70,000 known species of ostracods, both prehistoric and modern. They are among the most common fossils found, some of which are 450 million years old.

"They are an important group in that their remains store information about the environment they lived in," Renate Matzke-Karasz, a professor at the Ludwig Maximilian University of Munich in Germany and the lead author of the study (whose work with ostracod fossils in Australia goes back several years) said in a 2009 report on ancient shrimp. "The fossilized shells of ostracods are therefore a kind of archive of earth's history, storing information on climate, ecology and geology thousands, even millions of years ago."

Scientists are not sure why ostracods have such large sperm, but they believe they could have evolved long ago in that manner as a kind of reproductive strategy.

A human sperm would have to measure 17 meters, or 55 feet, to be in proportion with those of some ostracods. It would take about 34,000 human sperm to match the body length of the average grown man.

“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. “The new evidence that they have been around for millions of years only adds to the mystery.”