A sperm found fossilised in amber 100 million years ago may be the oldest ever discovered, scientists have revealed.

An international team of palaeontologists unearthed the “spectacular find”, which was preserved inside a female crustacean.

They believe the mussel-like creature mated shortly before becoming trapped in the resin.

Their findings, published in Proceedings of the Royal Society B, provide "an extremely rare opportunity" to learn more about the evolution of the reproductive process, they said.

Scientists analysed a tiny piece of amber with 39 ostracods trapped inside (PA)

Until now the oldest known fossilised sperm was found inside a 50-million-year-old worm cocoon from Antarctica.

The crustacean, a new species of ostracod called Myanmarcypris hui, is thought to have lived in coastal and inland waters of what is now Myanmar.

It would have been surrounded by trees that released huge amounts of resin.

While a majority of male animal species, including humans, produce large quantities of very small sperm to increase chances of fertilisation, there are exceptions.

Some creatures, such as fruit flies and modern-day ostracods, produce a small number of oversized sperm, with tails several times longer than the animal itself.

In these cases, the researchers say, chances of fertilising an ovum can increase with the size of the sperm cell.

A 3D reconstruction of a female ostracod (PA)

Knowing more about how giant sperm cells evolved could shed light on what the researchers describe as “ancient and advanced instance of evolutionary specialisation”.

A team led by Dr Renate Matzke-Karasz, a geobiologist at Ludwig-Maximilians-Universitaet (LMU) in Munich, made the discovery by analysing 39 specimens of ostracods trapped in a tiny piece of amber using 3D X-ray reconstructions.

The researchers discovered the ripe giant sperm stored in a pair of receptacles inside the female ostracod – waiting for the eggs to mature – in what could also be the earliest direct evidence of a completed insemination.

Dr Matzke-Karasz said: “The most significant part of our story is that we can now show that using giant sperm for reproduction is something that can last long in Earth history.

“Previously, we were not sure if animals that ‘switched’ to using these giant sperm at a certain point in their evolutionary history are doomed to become extinct very quickly.

“After all, these are enormous costs for the animals: large sperm must be produced, the reproductive organs are much bigger than in other species, they take up a lot of space in the animal, and mating lasts long.

“This is a lot of biological energy that must be allocated to reproduction – so you might think that this doesn’t make sense from an evolutionary standpoint.

“But in ostracods, it seemed to work for more than 100 million years.”

She added: “From an evolutionary point of view, sexual reproduction with the aid of giant sperm must, therefore, be a thoroughly profitable strategy.”

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