The female ostracode was unearthed by an international team of paleontologists. They believe he mated shortly before he was 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 fossilized sperm resided inside a 50-million-year-old worm cocoon in Antarctica.
The crustacean, a new species called Myanmarcypris hui, is believed to have lived in the coastal and inland waters of what is now Myanmar, surrounded by trees that produce huge amounts of resin.
A team led by Dr Renate Matzke-Karasz, a geobiologist at Ludwig-Maximilians-Universitaet (LMU) in Munich, analyzed 39 ostracods trapped in a tiny piece of amber using 3D x-ray reconstruction.
The researchers found mature giant sperm stored in a pair of containers inside the female ostracod, while waiting for the eggs to mature, which they said could also be the first direct evidence of completed insemination.
Most animals produce large amounts of very small sperm to increase the chances of fertilization. But some, like modern fruit flies and ostracods, produce small numbers of oversized sperm, with tails several times longer than the animal itself.
In these cases, the researchers say, the chances of fertilizing an egg may increase with the size of the sperm. Understanding the evolution of these giant sperm can shed light on what the team described as “an ancient and advanced example of evolutionary specialization.”
Dr Matzke-Karasz said: “The most important part of our story is that we can now show that the use of giant sperm for reproduction is something that can go on for a long time in Earth’s history.
“Previously, we didn’t know if animals that have ‘switched’ to using these giant sperm at some point in their evolutionary history are doomed to become extinct very quickly.
“After all, these are huge costs for animals. Large sperm must be produced, reproductive organs are much larger than in other species, they take up a lot of space in the animal, and mating takes a long time.
“That’s a lot of biological energy that needs to be allocated for reproduction – so you might think that doesn’t make sense from an evolutionary standpoint.
“But in ostracods, it seemed to work for over 100 million years. “
She added: “From an evolutionary point of view, sexual reproduction using giant sperm must therefore be a very profitable strategy. “