Researchers have discovered the world’s oldest fossilized sperm. According to the new study, the cells date back 50 million years. The sperm fragments were found entrapped in the wall of a fossilized worm cocoon in Antarctica.  

Sperm cells are rarely found in the fossil record as they tend to be short lived and have delicate structures that don't fossilize well. Benjamin Bomfleur, lead author of the study, and his research team accidentally came across the incredible find during their expedition in Antarctica. He told The Guardian that they were initially analyzing the fragments to “get a better idea of the structure of the cocoon.”

“When we zoomed into the images, we started noticing these tiny biological structures that look like sperm,” he explained.

The research team from Sweden, Italy and Argentina used a scanning electron microscope to look at the specimen's surface. They also used a particle accelerator in Switzerland to examine the internal structure of the cocoon, which revealed the presence of the sperm cell fragments from a 50-million-year-old Clitellata.

The ancient worm is thought to have secreted the cocoon while mating. Egg and sperm would then be released into the cocoon and protected by surrounding material. The cocoon finally being sealed, it would take several hours for the surrounding material to harden. Biological materials would then become trapped in this wall. Researchers suggest that this is how the sperm fossil was able to form and be preserved for millions of years, similar to how amber traps and preserves insect and plant materials over millions of years.

“All we found are fragments of the drill-bit-shaped ‘head regions,’ ornamented midpiece regions that presumably contain the nuclei, and very long, whip-like tails, some attached to the midpiece regions,” Bomfleur told Live Science.

The findings, detailed in The Royal Society journal Biology Letters, suggest that this specimen is 10 million years older than the next oldest known fossil of animal sperm. Bomfleur explained that the fossilized remains don’t contain any original organic material.

Researchers suggest that the sperm fragments are similar to that of a group of leech-like worms found on the shells of modern crayfish in the Northern Hemisphere, where they feed on dead organic matter. Further studies on the cocoon could reveal a great deal of information which, according to Bomfleur, has been overlooked by paleontologists who tend to focus on relatively hard structures like bones and shells.