Scientists believe they have uncovered the origins of sex.

In a study published in the journal Nature, an international team of scientists led by Prof John Long, from Flinders University in Australia, say that an ancient fish called Microbrachius dicki is the first known animal to reproduce by copulation rather than spawning.

The bony fish was around 8cm long and lived in lakes 385 million years ago in what is now Scotland.

The discovery was made when the researchers were analysing ancient fish fossils from Estonia and noticed a strange L-shaped appendage on a fossil of Microbrachius dicki, which they later determined was the male fish’s genitals.

The males had “large bony claspers” which delivered sperm along grooves to the female, which had a small bony structure on their rear to lock the male in place for copulation as the two fish were side-by-side.

Discussing how the fish would have mated, Long told The Conversation:

“It’s bizarre that these tiny fishes mated from a sideways position, the male and female resting alongside each other. They likely intertwined their bony jointed pectoral appendages (arms) using the rows of hooks on their inside edge. The outside arms could have helped them manoeuvre their large claspers into the mating position. With their hooked “arms” interlocked, the act of copulation in these fishes somewhat resembled square dancing the do-si-do.”