Scientists say they have found what may be the oldest specimen of fossilized sperm ever discovered, inside a tiny crustacean trapped in a piece of amber 100 million years ago.
The researchers say the discovery in amber from Myanmar's Kachin province, described in a paper published Wednesday in the science journal Proceedings of the Royal Society of Biological Sciences, provides an extremely rare opportunity to study the evolution of the reproductive process.
The scientists suspect the crustacean in which the sperm was found, a newly discovered species of ostracod about 1 millimeter long, was likely covered in amber shortly after mating.
They say the sperm cell found in the animal was significant, not only because of the age of the specimen but also because of its size — about one-fifth the size of the entire animal that produced it.
The researchers say that while most animals produce huge numbers of tiny sperm, there are still animals that exist today that produce so-called “giant” sperm. Some modern ostracods and species of fruit flies produce sperm many times longer than their bodies.
One of the authors of the study, the University of Munich’s Renate Matzke-Karasz, says the most significant aspect of the discovery is that it shows this method of reproduction has been around a very long time.
The researchers say it is unclear what evolutionary advantage producing a small number of giant sperm, as opposed to a large number of tiny sperm, may have. While a large sperm might have a better chance of reaching an egg, the reproductive organs of the animal producing them must be large as well, which would require a lot of “biological energy.”
Matzke-Karasz says that before this discovery, evolutionary scientists questioned whether animals that developed this type of reproductive system were doomed to extinction. Now, she says, they know they can exist for millions of years.