Sperm – the bane of mothers to teenage boys (and fathers to teenage girls, for that matter), it’s typically one thing – small. Microscopic, even. At least, it is for humans. That’s not the case for Miocene epoch seed shrimp, which evidently produced what scientists have identified as the oldest known fossilized sperm.

Though the tiny seed shrimp (which are actually bivalves) were just millimeters long, their sperm reached an incredible 0.4 inches in length. The fossilized sperm was found in a cave in Australia, where bat guano dropping into the water might have helped preserve the sperm.

What’s more, lead researcher Renate Matzke-Karasz, a geobiologist at Ludwig-Maximilian-University in Germany, says this may not by a relic of a bygone era. “But the most astounding aspect of our findings is that it strongly suggests that the mode of reproduction in these tiny crustaceans has remained virtually unchanged to this day,” she told LiveScience.

The 16 million year old sperm are unique in that they lack the “swimmers,” or tails normally associated with sperm. Instead, they have just the exceptionally elongated heads that ripple and rotate when organelles along its membrane are contracted. This is the only way the structure was able to move.

The seed shrimp cells predate the previous oldest known ostracod sperm cells by many millions of years, with the previous example only being a few thousand years old. Though perhaps among the oldest, and certainly large, the ancient seed shrimp comes up short in the competition for longest sperm: That honor goes to Drosophila bifurca, a modern fruit fly whose seed stretches to more than 2 inches.

The full findings are published in the journal Proceedings of the Royal Society B.