A team of palaeontologists have discovered what they believe is the world's oldest animal sperm, frozen in tree resin 100 million years ago inside a tiny crustacean in Myanmar.

The oldest known examples of fossilised animal sperm were previously a mere 17 million years old, according to the team of experts led by Wang He of the Chinese Academy of Science in Nanjing.

The sperms were found inside an ostracod -- a type of crustacean that has existed for 500 million years and can be found in many oceans today, they said in a paper published on Wednesday in the Royal Society's Proceedings journal.

They were found in the body of a female specimen, indicating that she must have been fertilised shortly before being trapped in amber, the experts said.

The individual sperms were described as "giants", measuring up to 4.6 times the size of the body of the male. 

"This is equivalent to about 7.30 metres (24 feet) in a 1.70-metre human, so it requires a lot of energy to produce them," Renate Matzke-Karasz of the Ludwig Maximilian University of Munich, co-author of the study, told AFP. 

The ostracod was also a new species that the scientists have named "Myanmarcypris hui".

'Eureka moment' 

Fossilised shells of ostracods are common but finding a specimen with "soft parts" is rare, the experts said.

During the Cretaceous period around 145 to 66 million years ago, the ostracods in question probably lived in the coastal waters of present-day Myanmar where they became trapped in a blob of tree resin.

The specimen was handed over to researchers by a Chinese collector in 2017. 

The tiny creatures are less than a millimetre long but the scientists made 3-D reconstructions of them to observe them more closely, leading to what Wang described as "one of those special Eureka moments in a researcher's life". 

As well as the sperm, the reconstructions also showed the distinctive muscular sperm pumps and penises (two of each) that male ostracods use to inseminate the females.

"This specimen allowed us to confirm our hypothesis that such giant sperm cells existed 100 million years ago," Matzke-Karasz said. 

Until now, this theory had been based on the discovery in 2009 of large genital organs in the crustaceans, which suggested the existence of correspondingly large sperms.

Most males in the animal world including humans produce tens of millions of tiny sperm cells, but ostracods are different -- for them, it's all about quality over quantity.

There are several conflicting theories about the evolutionary value of such giant sperms.

"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," said Matzke-Karasz.

'Co-evolution'

The researcher believes the giant sperm are a sign of good health on the part of the males, a characteristic "favoured" by the females, whose genitals evolved to accommodate them in an example of "co-evolution". 

"Given that many ostracods can reproduce parthenogenetically, without the need for males, sexual reproduction with giant sperm cells must have a clear advantage over asexual reproduction," Matzke-Karasz said. 

"To show that using giant sperms in reproduction is not an extinction-doomed extravagance of evolution, but a serious long-term advantage for the survival of a species, we need to know when they first appeared."

Scientists have been researching the amber of Myanmar for decades, finding all kinds of frozen treasures including frogs, snakes and a feathered dinosaur tail.