The sperm was found in the fossilised egg case of a prehistoric earthworm or leech-like creature. The discovery of the sperm, which the researchers say is around 50 million years old, is particularly remarkable because they know so little about the creatures who the egg casing (and the sperm!) belonged to.
Earth worms, leeches and their relatives belong to an animal group called Clitellata. Their bodies are made entirely of delicate soft tissues which are only fossilised in exceptional conditions. With their remains so rarely leaving a trace in the fossil record, scientists have very few historic records to study and so know little about their origin and evolution.
The only trace these organisms do leave in the fossil record are their egg cases. The cocoon-like structures, produced from mucous, are extremely resilient to physical and chemical decay and so often stick around long enough to be fossilised.
Scientists analysed an egg casing found in Antarctica under a scanning electron microscope and saw tiny structures which have similarities to the structures found in the sperm of ‘cray fish worms’ - living relatives of the Clitellata family group.
When egg cases are made the hermaphrodite adult clitellate will withdraw from the cocoon releasing both an egg and sperm before sealing the casing. Sperm can become trapped in the protein-packed cocoon wall before it entirely solidifies leaving a rare trace of the worm sperm to be fossilised.
Wider studies of fossilised egg casings could reveal more clitellate secrets, offering scientists an opportunity to learn more about the evolution of these creatures.