Researchers have discovered the world's oldest sperm cells preserved in a tiny cocoon in Antarctica for a whopping 50 million years, revealed a report published in Biology Letters. The study said that an ancient relative of worms or leeches likely created the cocoon while mating, and released its sperm inside.
Lead author Benjamin Bomfleur, paleontologist at the Stockholm-based Swedish Museum of Natural History, said, "Our discovery of sperm in a leech cocoon from Antarctica is the oldest record of fossil animal sperm and one of only a tiny number of such fossils in the geological record. The researchers found the cocoon while sieving sediments for small vertebrate remains during an expedition in Antarctica."
The research team picked individual cocoons from dry-sieved sediment samples of poorly consolidated, shelly conglomerate. The fossils were analyzed via light and scanning electron microscopy and via synchrotron-radiation-based X-ray tomographic microscopy (electronic supplementary material).
Bomfleur said, "Although it is a challenge to compare sperm fragments to the sperm of modern species, the drill-bit-shaped head regions do appear strikingly similar to those of this one peculiar group of leech like worms that is today only found living symbiotically on crayfish in the Northern Hemisphere. The next-oldest known fossil of animal sperm date back to about 40 million years ago."