Scientists accidentally discovered a new animal sperm fossil in Antarctica that dates to 50 million years ago. The sample, which is beleived to originate from an ancient Antarctic worm or leech, dethrones the 30-million-year-old mussel shrimp sperm fossil as title holder of the oldest animal sperm fossil ever discovered.

Thomas Mörs, a senior curator of the Swedish Museum of Natural History (SMNH), was examining rock samples from Antarctica in some animal remains when he spotted the fossilized worm cocoons. Using an electron microscope, he discovered that the cocoons have numerous sperms in them.

Further analysis revealed that the ancient sperm matches the characteristics of sperm from the modern-day crayfish worm, which is related to earthworms and leeches that feed on dead organic matter.

"Surprisingly, modern crayfish worms are only known from the Northern Hemisphere,” Steve McLoughlin, study co-author and a senior curator at SMNH, said to the National Geographic. "If our identification is correct then it implies that this group of animals had a much greater geographic range [50 million years ago] than they do today.”

Researchers believe the sperm got trapped inside the cocoons when it was released during mating before the enclosure's walls hardened. The cocoons helped preserve the fossils for millions of years. The discovery could provide insight into ancient worms and their ecosystem.

"Because sperm cells are so short-lived and fragile, they are vanishingly rare in the fossil record," Benjamin Bomfleur, study lead author and a paleontologist at SMNH, told LiveScience. "Our discovery of sperm in a leech cocoon from Antarctica is the oldest record of fossil animal sperm and one of only a tiny number of such fossils in the geological record."

The study was published in the July 15 issue of Biology Letters.