Scientists have accidentally discovered the oldest fossil animal sperm ever found.

The sperm cells appear to have been left behind by a pair of slimy, squirmy leeches at the edge of the world 50 million years ago. The pair of mating leeches lived on what is now Seymour Island in Antarctica during the Eocene, about 16 million years after dinosaurs went extinct.

That makes them quite a lot older than the 17-million-year-old giant fossil sperm of crustaceans called ostracods that was named as the oldest sperm in the world in 2014.

Thomas Mors, a paleontologist at the Swedish Museum of Natural History, had been on Seymour Island with a Swedish and Argentine team, looking for fossils of tiny, early mammals that once lived in Antarctica in a moist, temperate forest near the coast.

Researchers Seymour Island

The fossils were found on Seymour Island in Antarctica by a team of Swedish and Argentine paleontologists, including Thomas Mors who had been looking for mammal fossils. (Thomas Mors/Swedish Museum of Natural History)

As the researchers sieved the sediments they found some barrel-shaped beads a few millimetres across, recalled colleague Steve McLoughlin.

Thinking they were seeds, Mors sent them to McLoughlin and Benjamin Bomfleur, who work as paleobotanists at the museum.

McLoughlin and Bomfleur recognized them as worm or leech cocoons, which they had previously found in fossil plant samples.

Cocoon

Paleontologist found barrel-shaped beads a few millimetres across in the fossil sediments and thought they were seeds. They turned out to be leech cocoons. (Steve McLoughlin/Swedish Museum of Natural History)

When worms or leeches mate, they secrete a jelly around the fertilized eggs that hardens into a protective cocoon. Sometimes microorganisms such as tiny worms or bacteria can get trapped in the cocoon walls as they harden. The researchers decided to put the fossil cocoons under an electron microscope to see if they could find anything trapped in the walls.

"And then we found that the walls were full of these little spermatozoa," McLoughlin said in a phone interview. "We were surprised because they're exceedingly short-lived and very delicate, so it's unusual to find them."

Long drill bit

The scientists looked at previous studies on leech sperm and discovered that each species of leech had different sperm, and they could be used to identify the leech.

In this case, the "drill bit" structure originally attached to the head of the sperm to pierce the egg cell were unusually long, helping identify them.

The researchers think they were the sperm of a crayfish worm, a freshwater leech that attaches itself to the body of crayfish and scavenges any bits that the crayfish leaves behind as it eats.

McLoughlin said modern crayfish worms are only found in the Northern Hemisphere, but the new discovery suggests that they once had a much bigger range.

The findings were published this week in the journal Biology Letters.

In addition to the sperm, the researchers also spotted some bean-like microbes trapped in the cocoon walls that they think may be bacteria.

"There are potentially a whole lot of other little soft-celled microorganisms that can get entombed in these structures, just like amber," McLoughlin said. "That's something that we're going to look at in the future."

Leech cocoon

When worms or leeches mate, they secrete a jelly around the fertilized eggs that hardens into a protective cocoon. This diagram shows how the sperm and microorganisms can become encased in the process. (Steve McLoughlin/Swedish Museum of Natural History)