Fossilized semen discovered inside a mussel-like crustacean caught in amber 100 million years ago may be the oldest ever found, researchers say.
The female ostracod was found by an international team of paleontologists. They believe it was mated shortly before they were trapped in the resin.
Their findings, published in Procedures of the Royal Society B, provides “an extremely rare opportunity” to learn more about the evolution of the reproductive process, they added.
Until now, the oldest known fossilized seed has been fortified inside a 50-million-year-old worm cocoon from Antarctica.
The crustacean, a new species called the Myanmarcypris hui, is thought to have lived in coastal and inland waters of what is now Myanmar, surrounded by trees that produced enormous amounts of resin.
A team led by Dr. Renate Matzke-Karasz, a geobiologist at the Ludwig-Maximilians-Universitaet (LMU) in Munich, analyzed 39 ostriches caught in a small piece of amber using 3D X-ray reconstruction.
The researchers found mature giant sperm stored in a few containers inside the female ostracod, waiting for the eggs to mature, in what they said could also be the earliest direct evidence of a completed insemination.
Most animals produce large amounts of very small sperm to increase the chances of fertilization. However, some, such as fruit flies and modern ostracodes, produce a small number of excessive sperm with tails several times longer than the animal itself.
In these cases, the researchers say that the chances of fertilizing an egg may increase with the size of the sperm. Understanding the evolution of such a giant sperm can shed light on what the team described as “ancient and advanced occurrences of evolutionary specialization.”
Dr Matzke-Karasz said: “The most important part of our history is that we can now show that the use of giant sperm for reproduction is something that can last a long time in the history of the Earth.
“In the past, we were not sure whether animals that ‘switched’ to use this giant sperm at some point in their evolutionary history are doomed to extinction very quickly.
“After all, this is a huge cost for the animals. Large sperm must be produced, 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.
“This is a lot of biological energy that needs to be allocated to reproduction – so you might think that this does not make sense from an evolutionary point of view.
“But in ostracods, it seemed to work for more than 100 million years.”
She added: “From an evolutionary point of view, sexual reproduction using giant sperm should therefore be a thoroughly profitable strategy.”