The discovery was made by researchers from the Queen Mary University of London and the Chinese Academy of Science in Nanjing.
They discovered the sperm in a newly identified species of crustacean which has been preserved inside a piece of amber – fossilised tree resin – that was formed during the Cretaceous period.
It’s believed the animals, which have been named Myanmarcypris hui, had sex just before their entrapment in the amber.
Specimens of fossilised sperm are exceptionally rare.
The oldest previously known examples are around 17 million years old.
Myanmarcypris hui is an ostracod – a kind of crustacean that has existed for 500 million years and lives in all kinds of aquatic environments, from deep oceans to lakes and rivers.
Fossilised ostracod shells are common and abundant but finding specimens preserved in ancient amber with their appendages and internal organs intact provides a rare opportunity to learn more about their evolution.
Professor Dave Horne, professor of micropalaeontology at Queen Mary University of London, said: “Analyses of fossil ostracod shells are hugely informative about past environments and climates, as well as shedding light on evolutionary puzzles, but exceptional occurrences of fossilised soft parts like this result in remarkable advances in our understanding.”
During the Cretaceous period the ostracods were likely to have been living in a coastal lagoon fringed by trees in what is now Myanmar when they became trapped in a blob of resin.
Using X-ray microscopy the team created computer-aided 3D reconstructions of the ostracods, which measure less than 1mm long.
“The results were amazing – not only did we find their tiny appendages to be preserved inside their shells, we could also see their reproductive organs,” He Wang said.
“But when we identified the sperm inside the female, and knowing the age of the amber, it was one of those special ‘eureka’ moments in a researcher’s life.”
Myanmar amber has previously yielded other ground-breaking prehistoric finds, including frogs, snakes and a feathered dinosaur tail.
The team found adult males and females within the amber, but it was a female specimen that contained the sperm – indicating it must have mated shortly before becoming trapped in the amber.