The world's oldest sperm cells preserved in a tiny cocoon in Antarctica for a whopping 50 million years have been recovered by researchers, a scientific journal has reported.

An ancient relative of worms or leeches likely created the cocoon while mating, and released its sperm inside, the study 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," lead author Benjamin Bomfleur, paleontologist at the Stockholm-based Swedish Museum of Natural History, was quoted as saying by Live Science.

The discovery was reported in the journal l Biology Letters. The researchers found the cocoon while sieving sediments for small vertebrate remains during an expedition in Antarctica, Bomfleur added.

Benjamin Bomfleur said that the discovery happened by accident. While on an expedition on Seymour Island on the Antarctic Peninsula, Thomas Mörs, also a paleobiologist, was looking for small mammal bones when he came across a fossilised cocoon, The Independent reported.

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Individual cocoons were picked from dry-sieved sediment samples of poorly consolidated, shelly conglomerate.

The fossils were analysed via light and scanning electron microscopy and via synchrotron-radiation-based X-ray tomographic microscopy (electronic supplementary material).

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 leechlike worms that is today only found living symbiotically on crayfish in the Northern Hemisphere", Bomfleur said.

"The next-oldest known fossil of animal sperm date back to about 40 million years ago," Bomfleur added.

Inputs IANS