Ancient sperm have been discovered in Antarctica, hidden inside the cocoon of a strange worm who lived long ago. These reproductive cells are around 50 million years old, making them the oldest sperm remnants ever recovered, researchers report.

Thomas Mörs from the Swedish Museum of Natural History was searching the icy surface of Antarctica for mammal remains when he came across the fossilized cocoon. At first, the researcher believed the artifact contained just plant fossils, rather than anything more exotic. He passed the artifact onto a colleague named Benjamin Bomfleur, who recognized sperm hidden within the rocky matrix. Although the fossils are remarkably well preserved, the artifact contains no genetic material.

The early Eocene period, to which the sperm fossils date, was a time in the Earth's past marked by the rise of the world's first horses, sheep, and rhinos. At this time, Antarctica would have been significantly warmer than today, its climate possibly similar to that of present-day southern Chile.

Researchers believe the sperm may have been preserved for so long because the material became lodged in the sticky innards of the cocoon before the structure fossilized. This process is similar to an insect becoming trapped in amber.

Sperm cells are notoriously short-lived and fragile, so these structures are only rarely preserved in the fossil record. Prior to this discovery, the oldest known animal sperm fossils were dated to just 40 million years before our time. Discovery of  fossilized sperm are exceptionally rare, and this finding could suggest further finds to come. Leeches, earthworms, and some other similar animals create cocoons which can contain sperm and eggs released into the surroundings.

"The way that the cocoons harden over the course of a few days means that not only sperm cells, but also a host of other microscopic organisms, can become fossilized in the cocoon walls in a similar manner to bugs being entombed in amber," Benjamin Bomfleur from the Swedish Museum of Natural History said.

The worm itself was a member of the Clitellata class of worms, part of the Annelid phylum, and investigators believe it looked much like it's modern-day descendants. Microscopic analysis of the sperm revealed these structures also largely resembled modern-day counterparts. Heads of the the cells resemble drill bits, similar to the male reproductive cells of crayfish worms, which are native to the Northern Hemisphere.

"Spermatozoa, being very transient and delicate, are hardly ever preserved in the fossil record. We have uncovered a new type of medium which we think will hold great potential for similar findings in future studies," Bomfleur said.

Discovery of the ancient fossilized sperm was detailed in the Royal Society journal Biology Letters.