Evidence of ancient bony fish suggests that they copulated even though many of their descendants stopped doing so. Paleontologists believe this is the first-known animal to stop reproducing by spawning and instead mate by having sex.

Photo credit: B Choo, Flinders University.

The primitive fish (Microbrachius dicki) measured about 8cm long and is now long extinct.  As it often is today, the first sexual activity was a bit awkward. Constrained by their anatomy, the fish probably had to mate side by side.

“They couldn’t have done it in a ‘missionary position’,” said Prof Long. “The very first act of copulation was done sideways, square-dance style.”

But they also had something to help them – small arm-like fins, which could allow them to maintain the same position.

“The little arms are very useful to link the male and female together, so the male can get this large L-shaped sexual organ into position to dock with the female’s genital plates, which are very rough like cheese graters. They act like Velcro, locking the male organ into position to transfer sperm.”

Image credits: John Long, Brian Choo/Flinders University

During the Devonian, almost 400 million years ago, bony fish called placoderms dwelled in lakes in what is today’s Scotland.  A thorough study of fossils of these armour-plated creatures, which gave rise to all current vertebrates with jaws, suggests that their descendants — our perhaps even ancient ancestors — switched their sexual practices from internal to external fertilization. But the practice was not successful at first – so the descendants of the first animals which practiced internal copulation swtiched back to external fertilization.

Microbrachius dicki fossils are common – but nobody noticed the sexual organs until now. Image credits: Prof John Long

“This was totally unexpected,” says John Long, a palaeontologist at Flinders University in Adelaide, Australia, and lead author of the study, published in Nature. “Biologists thought that there could not be a reversion back from internal fertilization to external fertilization, but we have shown it must have happened this way.”

It took several million years before internal copulation was redeveloped, by the ancestors of today’s sharks and rays. Fossils of Microbrachius dicki are quite common, but interestingly enough, no one observed their reproductive organs.

Commenting on the research, Dr Matt Friedman, from the University of Oxford, UK, said:

“The placoderm group (which includes Microbrachius dicki) is a well known group – the fossils are pretty common, and it’s not as if this one was found in some far-off, exotic part of the world. It was found in Scotland. It is very remarkable that we haven’t noticed this before.”