We’ve found the world’s oldest sperm. And it’s big.

Science News reports on an ostracod fossil that’s 100 million years old – and contains a a recognizable sample of its giant sperm:

A single piece of amber from Myanmar held 39 ostracods, including many from a newly discovered species, Myanmarcypris hui. Using micro-CT scans, Dave Horne, a micropaleontologist at Queen Mary University of London, and colleagues peered inside a few of the tiny shelled animals.

“We knew from looking at the piece of amber with an ordinary light microscope that there were antennae and legs sticking out of the shell, so we were hopeful of finding internal organs,” Horne says. If delicate parts like legs and antennae are preserved, it’s likely that other soft parts are too, he says. “But what we saw … exceeded expectations.”

In ostracods, giant sperm possibly resulted from “competition between sperms of two or more males trying to fertilize the eggs of the same female,” Horne says. “This must be a highly successful reproductive strategy to have lasted for a hundred million years.” Ostracod sperm must make a long, winding journey from the female’s vagina to her eggs, adds study coauthor Renate Matzke-Karasz, a geobiologist at Ludwig Maximilians University Munich. Spirals in the canal to the eggs make the distance longer than the entire length of the female. Shorter sperm might not be able to make the journey, Matzke-Karasz says.

Finding ancient preserved sperm is rare, as soft tissues often decompose. But a series of recent discoveries — 17-million-year-old ostracod sperm reported in 2014 and 50-million-year-old worm sperm described in 2015 — has expanded the sperm fossil record. The new find rivals the age of reported fossilized sperm found in crickets from the Cretaceous Period. But Horne, Matzke-Karasz and colleagues argue that the newfound ostracod sperm is “the oldest unequivocal fossil animal sperm.”

There are colorized images at the link, and you can read the Proceedings of the Royal Society paper here.