Top Image: Scientists have found the world’s oldest sperm cells.

Paleontologists working in Myanmar have found the oldest sperm cells in the world. They were trapped in amber 100 million years ago, have been described as ‘giant,’ and come from a mussel-like crustacean. The creature housing the reproductive cells has also been declared a unique species and a fantastic find for researchers interested in the evolution of sex.

The discovery of the world’s oldest sperm cells has been reported by an international team of scientists in the journal Proceedings of the Royal Society B . They state that the reproductive cells were found inside a female specimen of a tiny crustacean from the Cretaceous period (about 100 million years ago). Science Daily declares that this crustacean contains “giant sperm cells in its reproductive tract” and is “the oldest fossil in which sperm cells have been conclusively identified.”

One of the ostracods trapped in amber.

This miniscule creature is known as an ostracod. Ostracods have been around for about 500 million years, and there are thousands of ostracod species. Their bodies, which range in size from 0.2-30 mm, are protected by a pair of a calcareous valves, which could remind you of a mussel shell

The female ostracod had been fossilized alongside others in amber that was found at a site in the province of Kachin in Myanmar. Previous fossilized remains found trapped in amber in the area include frogs, snakes, and part of a supposed dinosaur.

Beside a few insects, 39 ostracod crustaceans were entrapped in this tiny piece of Cretaceous amber found in Myanmar, including one containing the world’s oldest sperm cells.

X-Ray’s Revealed the World’s Oldest Sperm Cells

Scientists analyzed the remains of the ostracods inside the amber by using computer-assisted 3D X-ray reconstructions. Science Daily reports that this enabled them to reveal the tiny limbs and even the reproductive organs of the crustaceans with “astonishing detail.” This is how study author Dr. Matzke-Karasz and colleagues discovered the oldest sperm cells known to date.

He Wang of the Chinese Academy of Sciences in Nanjing explained that the cells were found within the female’s sperm receptacles, where they were stored while waiting for the eggs to mature. “This female must have mated shortly before being encased in the resin,” He Wang said.

The X-rays also showed the scientists the sperm pumps and the pair of penises male ostracods insert into the twin gonopores of the females during reproduction. According to the Independent, the researchers believe this may also be “the earliest direct evidence of a completed insemination.”

A 3D reconstruction of the female ostracod.

Finding fossilized sperm cells is uncommon. The article published in Proceedings of the Royal Society B states that the results “also double the age of the oldest unequivocal fossil animal sperm” (previously the oldest ostracod sperm are 17 million years old ), and highlight “the capacity of amber to document invertebrate soft parts that are rarely recorded by other depositional environments.” According to the Independent, the second oldest known fossilized sperm “resided inside a 50-million-year-old worm cocoon from Antarctica.”

The Same Sex Strategy for 100 Million Years

Apart from discovering the world’s oldest sperm cells, the researchers write in their study that their results also “reveal that the reproduction behavioural repertoire, which is associated with considerable morphological adaptations, has remained unchanged over at least 100 million years—a paramount example of evolutionary stasis.”

Specifically, the study results show that the generation of giant sperm (oversized sperm with tails that are often much larger than the animal itself) can increase the chance of a fertilizing an ovum. Fruit flies and ostracods have developed this method to increase their chances of reproduction, while the males of most animal species (such as humans) produce larger numbers of smaller sperm as part of their reproductive strategy.

Artist’s reconstruction of the Cretaceous ostracod crustacean Myanmarcypris hui male (right) and female (left) during mating.

By studying the evolution of giant sperm and its continuity over the ages, researchers can learn more about the “ancient and advanced instance of evolutionary specialization .” As Dr. Matzke-Karasz explained:

“The most significant part of our story is that we can now show that using giant sperm for reproduction is something that can last long in Earth’s history. Previously, we were not sure if animals that 'switched' to using these giant sperm at a certain point in their evolutionary history are doomed to become extinct very quickly. After all, these are enormous costs for the animals. Large sperm must be produced, the reproductive organs are much bigger than in other species, they take up a lot of space in the animal, and mating lasts long.”

Dr. Matzke-Karasz also notes that this biological energy requirement for reproduction may not seem to “make sense from an evolutionary standpoint” at first glance. But the fact that giant sperm was developed at least 100 million years ago and continues to be present in some species today is “a pretty impressive record for a trait that requires a considerable investment from both the males and females of the species. From an evolutionary point of view, sexual reproduction with the aid of giant sperm must therefore be a thoroughly profitable strategy.”