Researchers from the Swedish Museum of Natural History have discovered sperm cells preserved inside a fossilized worm cocoon aged 50 million years.

Benjamin Bomfleur and colleagues saw fragments of the oldest animal sperm in the world after examining samples of the cocoon fossil's inner surface using an electron microscope. Taken from Seymour Island off the Antarctic Peninsula, the fossil was collected by an expedition from Argentina. Sperm cells are delicate so it is rare for them to be found alongside fossils. Before this discovery, which was published in the journal Biology Letters, the oldest sperm on record was from springtails set in Baltic amber, aged around 40 million years old.

The researchers had a bit of a juvenile moment, at first, laughing at their findings, but they soon realized that it made sense to discover sperm where they did because some worms (as well as leeches and earthworms) secrete cocoons and deposit eggs and sperm inside the pouches. Each cocoon eventually hardens, forming a protective cover for the embryo developing inside.

It remains unclear to what kind of worm the sperm belongs to but the helical structures that look like beaded tails and drill bits characteristic to sperm from crayfish worms, which are leech-like in appearance and thrive on freshwater lobsters.

However, crayfish worms are only seen in the Northern Hemisphere, making it unlikely that the worm lived 50 million years ago in Antarctica. The researchers said that it was possible though that the crayfish worm has a distant relative that produced similar sperm.

No DNA was also present to be extracted since the cocoon's original composition would have changed a long time ago. The researchers are confident though that the cells they saw were not simply mineralized casts of the cells' shape but are rather actual imprints of the inner structure.

Sperm cells have been the subject of studies for decades as a means of understanding evolutionary relationships in worm species. Had the researchers had better equipment at their disposal, then they would've been able to reveal more details about the sperm cells they found, offering better insights on the discovery.

Thomas Mörs, Stephen McLoughlin, Marco Ferraguti and Marcelo A. Reguero also contributed to the research.