According to reports, researchers have discovered sperm cells that are 50 million years old and with these have become the oldest known animal sperm cells. Researchers made the discovery during an Antarctic expedition.

According to the researchers, the sperm fragments were embedded inside the walls of a fossilized cocoon. The researchers said they think that a prehistoric relative of leeches or worms could have formed the cocoon while mating, and released its sperm within it.

The researchers added that the sperm was trapped in the cocoon prior to hardening of the enclosure's walls. According to the researchers, the cocoon had been preserving the sperm cells while fossilizing over such a long period, just like amber entraps and preserves insects.

According to lead author Benjamin Bomfleur, a paleontologist at the Swedish Museum of Natural History, "Because sperm cells are so short-lived and fragile, they are vanishingly rare in the fossil record. Our discovery of sperm in a leech cocoon from Antarctica is the oldest record of fossil animal sperm". He added that it is among small number of such fossils that are in the geological record.

The study has been published in the journal Biology Letters. Bomfleur said the cocoon was found by the researchers while they were sieving sediments for small vertebrate remains. After that, the research team made use of a scanning electron microscope to study surface of the fossil and the particles on it with help of high magnifications. He also said that the next-oldest known animal sperm's fossil dates to nearly 40 million years ago.