Oldest Animal Sperm Found Inside Fossilized Worm Cocoon

The remains of a preserved 50-million-year-old cell may provide clues to the evolution of earthworms and leeches

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A fossilized spermatozoon fragment; the scale bar is 1 micrometer long.

The remains of long, thin cells preserved inside the 50-million-year-old fossilized cocoon of an unknown worm species represent the oldest animal sperm ever found, say researchers at the Swedish Museum of Natural History in Stockholm.

Benjamin Bomfleur and his colleagues spotted the sperm fragments when they used an electron microscope to examine the inner surface of the cocoon fossil, which had been collected by an Argentinian expedition on Seymour Island, which lies off the Antarctic Peninsula. Their findings are published today in Biology Letters.

Because of their delicate nature, sperm cells are very rarely found in fossils. The previous oldest animal sperm was from springtails preserved in Baltic amber, about 40 million years old. (Plant sperm fossils go back further, to 400-million-year-old specimens from early land plants in Scotland.)

Surprising discovery

The researchers do not know what kind of worm left the sperm. Scanning electron microscope images show helical structures resembling drill-bits and beaded tails, which are characteristic of sperm produced by crayfish worms, leech-like creatures that live on freshwater lobsters. But these animals are found only in the Northern Hemisphere, so it would be surprising if they had existed in Antarctica 50 million years ago, Bomfleur says. “It could be an extinct relative with similar types of sperm.”

There will be no extractable DNA left in the sperm fragments, Bomfleur adds, because the chemical make-up of the organic material would have changed from its original composition over such a long time. But his team expect that the cells they saw under the microscope are not merely mineralized outer casts of the original cell’s shape, but will retain their inner structure.

Taxonomic tool

Jakob Vinther, who studies invertebrate evolution at the University of Bristol, UK, agrees that cocoon fossils could represent an underexplored avenue for understanding the origin of earthworms and leeches. “I think we might have a really interesting system here that can be sort of a hidden window to the past,” he says. “There could be a lot of potential hidden gems inside those cocoons.”

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