The team, led by researchers from Chinese Academy of Science, found the sperm in a new species of crustacean they named Myanmarcypris hui.
They predict that the animals copulated just before their entrapment in the piece of amber (tree resin), which formed in the Cretaceous period, about 100 million years ago.
The researchers noted that fossilised sperm are exceptionally rare with previously oldest known examples being only 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, they said.
Their fossil shells are common and abundant but finding specimens preserved in ancient amber with their appendages and internal organs intact provides a rare and exciting opportunity to learn more about their evolution, according to the researchers.
The study, published in the journal Royal Society Proceedings B, also has implications for understanding the evolutionary history of an unusual mode of sexual reproduction involving “giant sperm.”
The researchers noted that new ostracod finds may be extremely small but in one sense they are giants.
Males of most animals — including humans — typically produce tens of millions of really small sperm in very large quantities, but there are exceptions, they said.
Some tiny fruit flies and ostracods are famous for investing in quality rather than quantity, producing relatively small numbers of “giant” sperm that are many times longer than the animal itself, according to the researchers.
These giant sperms are a by-product of evolutionary competition for reproductive success, they said.
The new discovery is not only by far the oldest example of fossil sperm ever found but also shows that these ostracods had already evolved giant sperm, and specially-adapted organs to transfer them from male to female, 100 million years ago, the researchers explained.
The researchers noted that each ostracod is less than a millimetre long.
Using X-ray microscopy the team made computer-aided 3D reconstructions of the ostracods embedded in the amber, revealing incredible detail.
“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 He Wang of the Chinese Academy of Science.
“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,” Wang added.
The researchers found adult males and females but it was a female specimen that contained the sperm, indicating that it must have had sex shortly before becoming trapped in the amber.
The reconstructions also revealed the distinctive muscular sperm pumps and penises — two of each — that male ostracods use to inseminate the females, who store them in bag-like receptacles until eggs are ready to be fertilised, they said.
According to the researchers, such extensive adaptation raises the question of whether reproduction with giant sperms can be an evolutionarily-stable character.
“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” said Renate Matzke-Karasz of Ludwig-Maximilians-University in Germany.
This new evidence of the persistence of reproduction with giant sperm for a hundred million years shows it to be a highly successful reproductive strategy that evolved only once in this group, the researchers said.
This is quite impressive for a trait that demands such a substantial investment from both males and females, especially when you consider that many ostracods can reproduce asexually, without needing males at all, they said.
“Sexual reproduction with giant sperm must be very advantageous” says Matzke-Karasz added. PTI SAR SAR
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