Sperm from 17 million years ago has been discovered by archeologists in Queensland, Australia. The reproductive cells are remarkably larger than the tiny crustacean which produced them.
Cells were discovered in five fossilized specimens of shrimp, including one male and four females. One of the females was a different species than the four other shrimp.
The method by which this fragile material was preserved for so long is still unknown. However, researchers believe bat guano may have played a part in the fossilization process.
The actual length of the sperm cells could not be measured in the samples. However, similar crustaceans alive today produce sperm four-tenths of an inch long. This could be up to four times longer than the animal from which it came. Human sperm are roughly 0.02 inches in length, just five percent as long as the shrimp.
"The evolution and function of this highly unusual feature of reproduction with giant sperm are currently unknown," researchers wrote in the journal article announcing their findings.
The cells were detected in the shrimp through the use of high-intensity X-rays. Unlike human sperm, these cells have two flagella or tail. They are long tubes, looking like small pipes. One of the female fossils was found filled with sperm, suggesting she mated just before dying.
Muscle shrimps are also called ostracods. These animals possess little more than a head and a pair of shells for defense, similar to a clam. They still exist in waters around the world. These tiny Ostracods are occasionally called seed shrimp, and thousands of species of the animals have been recognized so far by biologists. They live off the deitrus or organic debris that floats through water and often live in moss.
They also have a highly-unusual mating process which can involve up to three "genders," male, female and asexual female. During the act of mating, the animals can employ up to two sets each of reproductive organs. They act together to assist the process.
This behavior may have played a part in the unusually-detailed preservation of the specimens. Investigators believe that guano, dropped from bats, may have provided chemicals needed for fossilization of the delicate cells.
Ancient sperm has been discovered before, including cells from insects preserved in amber. The oldest of these samples date from the Early Cretaceous period, which lasted about 100 million years ago. Turtles have also been fossilized while in the process of mating.
Investigation of the shrimp fossils and ancient reproductive cells was published in the journal Proceedings of the Royal Society B.
