World's oldest sperm discovered: 17-million-year-old cell is longer than its creator

The cell - above - had to be coiled up tightly to fit inside its shrimp hosts

Scientists have discovered the world’s oldest sperm: a ‘giant’ of a cell belonging to a species of tiny shrimp that lived 17 million years ago in what is now modern-day Australia.

The sperm is unusual not just for its incredible age but for being longer than the animal it was found in; it managed to fit inside the shrimps’ sexual organs only by dint of being tightly coiled up.

“These are the oldest fossilised sperm ever found in the geological record,” said Professor Mike Archer of the University of New South Wales in a press release.

Professor Archer had been working at the same location for many years – a remote fossil deposit in northwestern Queensland known as Riversleigh – and has previously discovered other prehistoric animals at the site including a giant, toothed platypus and a flesh-eating kangaroo.

However, he says he was still shocked by the discovery of the sperm nuclei, describing the find as “totally unexpected” and adding “it now makes us wonder what other types of extraordinary preservation await discovery in these deposits.”

An artist's impression of the site 17 million years ago (complete with defecating bats). Image credit: Dorothy Dunphy

The survival of the animals’ soft tissue over such a long period of time is also rare, and is thought to be thanks to a “steady rain of poo from thousands of bats” in the pool where the shrimp were discovered.

It's likely that it was the high level of phosphorous in the animals’ faeces that not only killed off the tiny shrimp in the first place, but that also helped with the mineralisation process that preserved their soft tissues, with similar examples of 'preservation-by-poo' found in France.

The fossils in Riversleigh belong to a species of freshwater crustaceans known as ostracods and were originally collected in 1988. Years later they were passed to European specialists who discovered the sperm and recently published their findings in the journal Proceedings of the Royal Society B.

A modern example of the ostracod crustacean. Image credit: Dr. Renate Matzke-Karasz

Once fully unravelled the length of the cells is thought to be around 1.3 millimetres – about the same size or slightly longer than the shrimps themselves. This may sound unusual but is in fact relatively common, with super-giant sperm thought to confer various evolutionary advantages in the biological arms race.

Fruit flies, for example, are just a few millimetres long but have sperm cells that measure six centimetres in length. This gigantism means that males can out-reproduce rivals by making sure there’s literally no space for any competing genetic material to get near the female’s egg.

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