Some 50 million years ago, an Antarctic worm made a valiant attempt at propagating the species, building a special sex cocoon and storing his sperm inside.That particular species of worm is now extinct, but the sperm-filled cocoon was fossilized, preserving it until the present day in Antarctic rock. 

This reproductive sample, embedded within ancient cocoon walls, represents the oldest sperm cells on earth, as scientists from Sweden, Italy, and Argentina write in Biology Letters. The researchers examined the cocoon’s contents using an electron microscope, and found a bevy of sperm cells inside. According to their analysis, the cells seem to have belonged to a worm related to the modern-day species Branchiobdellida. 

Prehistoric sperm has been discovered before, petrified in amber or fossilized inside animals, as was the case with a 16-million-year-old sample discovered in the reproductive tract of an ancient shrimp. The discovery of sperm inside a fossilized cocoon suggests that other delicate evidence of evolution may have survived the millennia inside similar structures, providing a potential gold mine of data about soft-bodied animals like worms that rarely become fossils. 

[h/t: National Geographic