A fossilized sperm cell (Department of Palaeobiology, Swedish Museum of Natural History)

Benjamin Bomfleur can’t help but laugh a little when talking about this latest, record-setting discovery.

“A 50-million-year-old worm sperm from Antarctica?” he said between chuckles. “Who would have thought that’s possible?”

And yet, what Bomfleur, of the Swedish Museum of Natural History, and other researchers stumbled upon was a serious discovery: the oldest animal sperm ever found.

As it goes with many great discoveries, this wasn’t what they were looking for. While on an expedition on Seymour Island in the Antarctic, Thomas Mörs, a paleobiologist at the same museum, was looking for signs of small mammal bones when he discovered a fossilized cocoon. Remembering that cocoons like this often carried notable remains of plants, he passed it along to Bomfleur, a fellow palaeobiologist. Bomfleur took it and started looking for plant remains. That’s when he noticed the long, fragile remains of what appeared to be a sperm.

No expert in sperm himself, Bomfleur sent pictures of the specimen along to Marco Ferraguti, who happens to be an expert in annelid sperm.

Through radiometric dating, it was determined that the cocoon and its contents were at least 50 million years old, making the sperm the “oldest fossil animal spermatozoa yet identified.”

After comparing the pictures with his “enormous collection” of sperm pictures, Ferraguti determined the specimen was likely the product of a “crayfish worm.”

Now retired, Ferraguti made a point of mentioning that he finds it amusing that he gets to be part of such an eye-catching study because, while in the field, he found it “really, really difficult to tell people the work [he does] is interesting.”

What allowed this sperm to survive for so long was the biology of its creator.

The ancient worm secretes a cocoon, about 2 millimeters around, which then typically serves as a protective housing for an egg and sperm when the worm reproduces. The cocoon is formed by a sticky mucus that takes several days to harden, but once it does, biological material, such as sperm, can be trapped along its walls.


General location of Cocoon fossil. (Federico Degrange)

This can preserve all kinds of different materials for centuries.

Bomfleur likened it to amber preservation, though it’s unlikely this could be used for the creation of a Spielberg-esque, Jurassic worm (sorry).

“These things are — they are flexible,” Mörs said, comparing the cocoon to similar fossils typically formed by plants. “I didn’t expect at the beginning that these were from animals.”

The downside, Bomfleur said, is that the sperm was not completely intact. It was broken into fragments, making it difficult to learn much about the anatomy of this specific worm.

However, the research team also found remnants of clam shells and small animals within this cocoon.

The evolutionary history of Citellata (earthworms, leeches, etc.) is largely unknown, the team wrote in their paper, which was published in Biology Letters this week. Finding remnants of this type of species in the Northern Hemisphere is particularly rare, the study said, indicating that the evolutionary history of these worms is more complex than once thought.

More importantly, Bomfleur explained that though worms themselves don’t fossilize well, sperm can be a better clue as to the creator of a given cocoon. This discovery should encourage future researchers to look for similar structures.

“I think we might have a really interesting system here that can be sort of a hidden window to the past,” Jakob Vinther, who studies invertebrate evolution at the University of Bristol, told Nature. “There could be a lot of potential hidden gems inside those cocoons.”

While fossilized sperms are a rare find, Seymour Island is a known hot spot of noteworthy fossils, Mörs said — he’s been there three times already. The only part of Antarctica that is completely free of ice cover, the island is a perfect location for extraction. Mörs said other fossils originating from the mainland of the continent often wash up onto the island, bringing a variety of fossils, including small mammal bones and shark teeth.

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