Mating female and male Microbrachius, by Brian Choo, Finders University

Scientists have long believed that prehistoric fish all mated externally by releasing eggs and sperm into the water. But a new discovery — “one of the biggest in the evolutionary history of sexual reproduction” — has led an Australian professor to conclude that Microbrachius dicki, prehistoric armored fish, actually had genitals, and were using them to copulate internally 385 million years ago.

Paleontologist and Flinders University Professor John Long made the discovery accidentally, while going through old boxes of fossils at the University of Technology in Tallinn, Estonia. He found that the M. dicki is the first species in which the male and female developed physical differences. Males had ”genital limbs” called claspers on either side of their bodies that were used to transfer sperm into females, and females had ”genital plates,” which are rough like cheese graters, to dock the male organs in place. Those claspers are now believed to be the oldest sexual organs, which, over hundreds of millions of years, would evolve into the penis.

By Brian Choo, Finders University

The M. dicki also have limblike protrusions on their upper bodies — hence their name Microbrachius, which means “little arms” — and scientists never understood what they were for. Now it’s clear that the M. dicki use them to hold on to one another during copulation. The findings, which were first published in the science journal Nature, also indicate the position in which the fish had sex: side by side, as if they were square-dancing.

Here’s a video of the fun: