Fossilized sperm found in mussel crustaceans that were caught in amber 100 million years ago may be the oldest ever found, scientists say.
The female shark was discovered by an international team of paleontologists. They believe that it was concluded shortly before it was trapped in the resin.
Their findings, published in Notices of the Royal Society B, provide an “extremely rare opportunity” to learn more about the evolution of the reproductive process, they added.
So far, the oldest known petrified sperm was in a 50-year-old cocoon of worms from Antarctica.
The crustacean, a new species called Myanmarcypris hui, is thought to have lived in coastal and inland waters in present-day Myanmar, surrounded by trees that produce huge amounts of resin.
A team led by Dr. Renate Macke-Karas, a geobiologist at the Ludwig-Maximilians-Universitaet (LMU) in Munich, analyzed 39 ostracods caught in a small piece of amber using a 3D X-ray reconstruction.
The researchers found mature giant sperm stored in a pair of vessels inside the female ostracod, awaiting the maturation of the eggs, which they said could be the earliest direct evidence of complete insemination.
Most animals produce large amounts of very small sperm to increase the chances of fertilization. But some, such as fruit flies and modern ostracods, produce small numbers of large sperm, with tails several times longer than the animal itself.
In these cases, the researchers say, the chances of fertilizing the egg may increase with the size of the sperm. Understanding the evolution of such giant sperm can shed light on what the team described as “an ancient and advanced example of evolutionary specialization.”
Dr Matske-Karas said: “The most significant part of our history 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 weren’t sure if the animals that” switched “to using these giant sperm at some point in their evolutionary history were doomed to disappear very quickly.
“After all, these are huge costs for animals. Large sperm have to be produced, the reproductive organs are much larger than in other species, they take up a lot of space in the animal and mating lasts a long time.
“It’s a lot of biological energy that needs to be distributed for reproduction – so you might think it doesn’t make sense from an evolutionary point of view.
“But the ostracods seemed to work for more than 100 million years.”
She added: “From an evolutionary point of view, sexual reproduction with the help of giant sperm must be a completely profitable strategy.”