50-Million-Year-Old Sperm Found in Antarctica

The 50-million-year-old sperm cell. Swedish Museum of Natural History

Things without bones don't fossilize that well. This includes sperm and worms, and most definitely worm sperm. But scientists have now discovered fossilized sperm from a worm-like creature in Antarctica that is 50 million years old, as revealed in a study published Wednesday in the journal Biology Letters.

The researchers, from the Swedish Museum of Natural History in Stockholm, made the unexpected find while examining a cocoon made by one of these ancient animals. These cocoons, or egg cases, are extremely resistant to decay. Embedded inside the inner wall of the cocoon, the scientists found, was what looked like a spermatozoa cell. On further investigation, they determined that's exactly what it is—and concluded that it was 10 million years older than any other fossilized sperm ever found.

It's unclear exactly what these creatures looked like or where they would be placed taxonomically, although their sperm resemble those of crayfish worms, leech-like animals that live on freshwater lobsters, according to the study.

Unfortunately, there were no DNA remains, so there's no chance for a Jurassic Park-style rebirth of these leechy fellows.

The scientists dated the find by using a method called strontium isotope dating. This technique looks at the relative ratio of chemical varieties, or isotopes, of the element strontium, a number that has changed very slowly but predictably over the past tens of millions of years.

It's likely that these cocoons, which are often found in the fossil record, could have further surprises in store. "I think we might have a really interesting system here that can be sort of a hidden window to the past," Jakob Vinther, an invertebrate evolution specialist at the U.K.'s University of Bristol, told Nature. "There could be a lot of potential hidden gems inside those cocoons."