The world's oldest sperm - 50 million years older than any other ever found - has been found encased in a piece of amber dug up in Myanmar.
The sperm and the reproductive organs it came in belonged to an ostracod that swam the world's oceans 100 million years ago, when dinosaurs stalked the land.
The sperm was found in the reproductive tract of a female that had clearly just mated.
Ostracods, also known as seed shrimps, are a family of crustaceans that are still around today. Some of them - including the one found in Myanmar, Myanmarcypris hui - reproduce using a kind of giant sperm unique to ostracods.
It's such a rare evolution that fossils are hard to come by - the previous oldest ostracod sperm ever found (in Australia) was dated to 17 million years ago.
Comparing the one from 100 million years ago and ostracods today, the sperm and reproductive organs appear unchanged - "a paramount example of evolutionary stasis" according to the study, published in Proceedings of the Royal Society B: Biological Sciences.
"These results also double the age of the oldest unequivocal fossil animal sperm."
The previous oldest example was 50 million years old, found in an Antarctic segmented worm cocoon in 2015.
"The appearance of a complex reproductive mechanism involving giant sperm improves mating success and may have been an important contributor to the late Mesozoic explosive radiation of the superfamily Cypridoidea, which today includes the vast majority of nonmarine ostracod species," the researchers said.
Myanmarcypris hui was previously unknown to science.