Scientists from Flinders University in Australia say they have identified the first example of penetrative sex in evolution. And wow, was it ever weird.

Illustration: Brian Choo/Flinders University.

The new study, which now appears in the journal Nature, describes the copulation technique of an ancient, armored fish called placoderms that lived about 385 million years ago in Scotland. Placoderms, a primitive jawed vertebrate, are the earliest vertebrate ancestors of humans. The study's lead author, John Long, discovered the mating abilities when he stumbled across a single fossil bone in an Estonian collection.

This video shows a simulation of the fish getting it on — and it could be considered a bit NSFW.

The male member of the species, Microbrachius dicki (yes, really), evolved bony L-shaped genital limbs called claspers that transferred sperm to females; in turn, females developed small paired bones that locked the male organs in place for mating. It's considered the first example of a reproductive technique in fish that doesn't involve spawning, and the first use of internal fertilization and copulation as a reproductive strategy known in the fossil record.

Long had this to say in a statement:

Microbrachius means little arms but scientists have been baffled for centuries by what these bony paired arms were actually there for. We've solved this great mystery because they were there for mating, so that the male could position his claspers into the female genital area. It was previously thought that reproduction spawned externally in water, and much later down the track in the history of vertebrate evolution. Our earlier discoveries published in Nature in 2008 and 2009 of live birth and copulation in placoderms concerned more advanced placoderm groups. Our new discovery now pushes the origin of copulation back even further down the evolutionary ladder, to the most basal of all jawed animals. Basically it's the first branch off the evolutionary tree where these reproductive strategies started.

Given the constrained anatomy of the fish, they resorted to the rather inelegant mating style.

"They couldn't have done it in a 'missionary position'," noted Long in a BBC article. "The very first act of copulation was done sideways, square-dance style. 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."

Interestingly, this copulation technique did not last. As fish evolved they reverted back to spawning. It took another few million years for copulation to return, reappearing in ancestors of sharks and rays.

[ BBC | Flinders University ]

All images: Flinders University.