Worm sperm have staying power. Embedded in the wall of a fossilized cocoon, mineralized sperm from what may have been a leechlike worm have stuck around for about 50 million years.
The ancient sperm fossils are the oldest yet reported. They predate the previous record holders — amber-preserved sperm from tiny jumping insects called springtails and petrified sperm from mussel shrimps — by at least 10 million years.
Researchers discovered the cocoon-entombed worm sperm by sifting through sediments from an Antarctic island. The cells’ drill bit–shaped heads and grainy texture resemble the sperm of Branchiobdellida, a group of squiggly little worms that crawl on freshwater crayfish today.
Sperm are short-lived and fragile, so preservation happens only rarely. But cocoons may offer an untapped resource for finding soft and delicate cells, and even tiny organisms, researchers suggest July 15 in Biology Letters.