Fossilised sperm discovered inside a mussel-like crustacean that was trapped in amber 100 million years ago may be the oldest ever found, scientists say.
The female ostracod was unearthed by an international team of palaeontologists. They believe it 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 added.
Until now the oldest known fossilised sperm resided inside a 50-million-year-old worm cocoon from Antarctica.
The crustacean, a new species called Myanmarcypris hui, is thought to have lived in coastal and inland waters in what is now Myanmar, surrounded by trees that produced huge quantities of resin.
A team led by Dr Renate Matzke-Karasz, a geobiologist at Ludwig-Maximilians-Universitaet (LMU) in Munich, analysed 39 ostracods trapped in a tiny piece of amber using 3D X-ray reconstruction.
The researchers found ripe giant sperm stored in a pair of receptacles inside the female ostracod, waiting for the eggs to mature, in what they said could also be the earliest direct evidence of a completed insemination.
Most animals produce large quantities of very small sperm to increase chances of fertilisation. But some, like 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. Understanding the evolution of such giant sperm may shed light on what the team described as "ancient and advanced instance of evolutionary specialisation".
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."