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Science Notebook

Why are scientists so excited about fossilized worm sperm?

Researchers from the Swedish Museum of Natural History have discovered the oldest fossilized animal spermatozoa.

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    A fossilized spermatozoon fragment; the scale bar is 1 micrometre long.
    Courtesy of Paleobiology Department/Swedish Museum of Natural History
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Often scientific discoveries are found by accident, as was the case with the recent discovery of a 50-million-year-old worm sperm in Antarctica.

Paleobiologists at the Swedish Museum of Natural History in Stockholm were on an expedition with other researchers at Seymour Island in Antarctica. Thomas Mörs of the museum was searching for mammal remains when he stumbled upon the fossil of a cocoon. Originally thinking it might contain plant remains, he passed it along to fellow researcher Benjamin Bomfleur who noticed strands of what appeared to be fossilized animal spermatozoa.

The oldest fossilized animal spermatozoa ever discovered.

“A 50-million-year-old worm sperm from Antarctica?” Dr. Bomfleur remarked to The Washington Post. “Who would have thought that’s possible?”

By comparing the cocoon and sperm cell characteristics under an electron microscope, they concluded that the sperm belonged to a species of annelid worms. After sending the sample to zoologist Marco Ferraguti, the team also concluded that the sample was around 50 million years old, making it the oldest animal sperm yet found.

After comparing the photos to those of living worms, Dr. Ferraguti, now retired, confirmed that they belonged to a type of “crayfish worm.”

“Surprisingly, modern crayfish worms are only known from the Northern Hemisphere,” said study-coauthor Steve McLoughlin, also at the Swedish Museum of Natural History, in an interview with National Geographic. “If our identification is correct then it implies that this group of animals had a much greater geographic range [50 million years ago] than they do today.”

Much is unknown about the history of Clitellata, which includes worms and leeches, and so this find could bridge many gaps. What makes this discovery even more interesting, as detailed in the study’s report in Biology Letters, is that fossilized sperm cells are rarely ever found. This is one of only a few cases in scientific history.

“Because sperm cells are so short-lived and fragile, they are vanishingly rare in the fossil record,” Bomfleur told National Geographic.

The reason this sample survived, according to the scientists, has to do with the material of the cocoon. These ancient worm cocoons, once hardened, can store material for centuries.

According to Bomfleur, the sperm was not completely intact when found, meaning the team won’t be able to learn as much as they could about the ancient species of annelid. The cocoon did, however, contain remnants of clam shells and other small animals.

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