Cells preserved inside a 50-million-year-old fossilized worm cocoon represent the oldest animal sperm ever found.

Credit: Swedish Mus. Nat. Hist./CC by 4.0

Because of their delicate nature, sperm cells are rarely found in fossils. But Benjamin Bomfleur at the Swedish Museum of Natural History in Stockholm and his colleagues spotted the sperm fragments (pictured) when they used an electron microscope to examine the inner surface of the cocoon fossil, which was found in Antarctica. Such cocoons are secreted by some worms, including earthworms and leeches, which deposit sperm and eggs inside.

The researchers do not know what kind of worm left the sperm. However, 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.

Biol. Lett. 11, 20150431 (2015)