(CN) — An international team of paleontologists uncovered the world’s oldest known sperm cells during an intensive examination of 100-million-year-old small crustaceans preserved in amber in what is now Myanmar.
The fossilized shell samples of minuscule crustaceans, which date back to the Cretaceous period, are common. Fossilized sperm cell samples, however, are extremely rare.
The samples also included a previously unknown species of crustacean, which scientists have named Myanmarcypris hui.
The new species is an ostracod, a type of crustacean also called seed shrimp that is identified by the dual calcareous valves that form its hard upper shell.
Ostracods have been around for 500 million years and can be found in oceans, freshwater lakes and rivers. But the specimens conserved in amber allow researchers the opportunity to examine details of their internal organs.
While closely examining the reproductive tract of a female crustacean, scientists found giant sperm cells, making the sample the oldest known fossil in which sperm cells have been identified, according to the study published Wednesday in the journal Proceedings of the Royal Society B: Biological Sciences.
The oldest known sperm cell sample prior to this discovery belonged to a species of worm that lived at least 50 million years ago and the oldest known ostracod sperm was 17 million years old, according to the study.
Renate Matzke-Karasz, a researcher at Ludwig Maximilian University in Munich, Germany, said in a statement the specimens found in the Burmese amber provide unprecedented insights into the ancient crustacean species.
“The finds gave us an extremely rare opportunity to learn more about the evolution of these organs,” said Matzke-Karasz. “The complexity of the reproductive system in these specimens raises the question of whether the investment in giant sperm cells might represent an evolutionarily stable strategy.”
Scientists used computer-assisted 3-D X-ray technology to examine the organisms and review details of their reproductive organs, including the male ostracod’s pair of penises, which are inserted into the twin gonopores of the female.
He Wang, researcher with the Chinese Academy of Sciences, said in a statement the sperm cells found in the female crustacean were likely stored after copulation with a male and were ready for release once the female’s eggs matured.
“This female must have mated shortly before being encased in the resin,” said Wang.
Most animal species — including humans — produce large numbers of small sperm cells. But the ostracod produces large sperm cells with motile tails that are several times longer than the organism itself.
“In order to prove that the use of giant sperm is not an extravagant whim on the part of evolution, but a viable strategy that can confer an enduring advantage that enables species to survive for long periods of time, we must establish when this mode of reproduction first appeared,” Matzke-Karasz said.
Matzke-Karasz said the production of large sperm cells by this ostracod 100 million years ago presents a deliberate evolutionary advancement.
“That’s a pretty impressive record for a trait that requires a considerable investment from both the males and females of the species,” said Matzke-Karasz. “From an evolutionary point of view, sexual reproduction with the aid of giant sperm must therefore be a thoroughly profitable strategy.”
Ostracods populated coastal and inland bodies of water in what is now Myanmar. In the Burmese province of Kachin, scientists have yielded massive troves of fossils of organisms conserved in the sticky resin produced by trees in the region, according to the study.
The newly uncovered species of ostracod is one of many organisms, including frogs and snakes, that were trapped in the oozing tree resin and preserved for millions of years.
In the last five years alone, hundreds of crustacean species have been newly described, challenging evolutionary biologists’ hypotheses on the relationship between ecological conditions and phylogenetics, or the evolutionary history of organisms.