Oldest fossil sperm found in Antarctica

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

The sperm cells appear to have been left behind bya pair of slimy, squirmy leechesat the edge of the world 50 million years ago. Thepair of mating leeches lived onwhat is now Seymour Island inAntarcticaduring 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 wasnamed 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 Argentineteam,looking forfossils of tiny, early mammals that once lived in Antarctica in a moist, temperate forest near the coast.

As the researcherssieved the sediments theyfound some barrel-shaped beads a few millimetres across, recalledcolleague Steve McLoughlin.

Thinking they were seeds, Morssent 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 previouslyfound in fossil plant samples.

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 findanything 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" structureoriginally 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 saidmodern crayfish worms are only found in the Northern Hemisphere, butthe new discovery suggests that they once had a much bigger range.

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

• Summary of the paper

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."