A partnership between researchers at Queen Mary University of London and the Nanjing Branch of the Chinese Academy of Sciences has led to the discovery of the world's oldest animal sperm.

The fossilized sperm was found inside tiny water creatures trapped in amber around 100 million years ago, in what is now Myanmar.

Paleontologists made the "spectacular find" preserved inside a female ostracod－a type of small crustacean that has been in existence for 450 million years.

The scientists believe that the animals were having sex just as they were enveloped by tree resin, which then fossilized into amber sometime in the Cretaceous period.

The study, published in the journal Royal Society Proceedings B, has implications for understanding the evolutionary history of the reproduction process in animals.

Dave Horne, professor of micropaleontology 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 fossilized soft parts like this result in remarkable advances in our understanding."

The experts said fossilized sperm samples are exceptionally rare; previously the oldest known examples were only 17 million years old.

Fossilized ostracod shells are common and abundant but it is rare to find specimens preserved in ancient amber with their appendages and internal organs intact.

They said this ostracod specimen belongs to a new species, named Myanmarcypris hui. The creatures lived in the coastal waters, where they became trapped in the blob of resin.

The team at the Chinese Academy of Sciences was able to use X-rays to obtain high-resolution images of the well-preserved soft parts of the ostracods.

They found "giant sperm" stored in a pair of sperm receptacles inside the female ostracod, waiting for the eggs to mature.

"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 "said Wang He, who led the study. "But when we identified the sperms inside the female, and knowing the age of the amber, it was one of those special Eurekamoments in a researcher's life".

There are conflicting theories on the evolutionary value of such long sperm, according to Renate Matzke-Karasz, a geobiologist at Ludwig Maximilian University of Munich, a co-author of the study who worked with the team at the Chinese Academy of Sciences.

"For example, experiments have shown that in one group, a high degree of competition between males can lead to a longer sperm life, while in another group, a low degree of competition also led to a longer sperm life," Matzke-Karasz told the Agence France-Presse.

She said the discovery shows "that reproduction with giant sperm is not an evolutionary extravagance on the brink of extinction, but a serious long-term advantage for the survival of a species".