The world’s oldest animal sperm fossil from 50 million years has been discovered by an international team of researchers in the wall of a fossilised worm cocoon in Antarctica. The findings have been published in The Royal Society journal Biology Letters.
A scanning electron micrograph of the 50-million-year-old fossilized sperm. Photo credits: Department of Palaeobiology/ Swedish Museum of Natural History/ Biology Letters 2015.
Sperm cells are normally short-lived – they do not usually stand the test of time and are hence only rarely found in fossils. Furthermore, their structure is delicate and cannot fossilise well. The discovery was, therefore, a great surprise to the scientists who accidentally stumbled upon it as they were working in Antarctica with the aim to exploring the structure of a worm (the Clitellata) cocoon.
“When we zoomed into the images, we started noticing these tiny biological structures that look like sperm,” Benjamin Bomfleur, the lead author of the study, said in a statement to The Guardian.
The surface of the specimen was analysed by a scanning electron microscope. Its internal structure was studied with a particle accelerator based in Switzerland. This is what revealed the sperm cell fragments embedded inside the cocoon of the 50-million-year-old Clitellata.
According to the researchers, the worm had secreted the cocoon while mating. As such, male and female gametes (sperm and egg) would be released into the cocoon which acted as a shield thereby protecting the cells from the outside world. The cocoon sealed, it was only a matter of hours for the surrounding material to harden. Consequently, biological substances locked inside would remain trapped into its wall. The scientists have thus explained the presence and preservation of the sperm fossil.
“All we found are fragments of the drill-bit-shaped ‘head regions,’ ornamented midpiece regions that presumably contain the nuclei, and very long, whip-like tails, some attached to the midpiece regions,” Bomfleur said in a statement to Live Science.
However, the fossilised remains do not contain any original organic material.
More studies have to be done to shed further light on the origin of the cocoon. According to Bomfleur, this would reveal a great amount of information.