Missionary impossible: How sex was a sideways story

Side order of sex: An artist's impression of antiarch fish mating. (Flinders University/YouTube)

Side order of sex: An artist's impression of antiarch fish mating. (Flinders University/YouTube)

Fossilised features of antiarch fish suggest that early intercourse was not the smoothest of affairs, with males faced with the task of steering their bony L-shaped organs between twin genital plates that adorned the females like tiny cheese graters.

“They could not have done it in the missionary position,” said John Long, professor of palaeontology at Flinders University in Adelaide, Australia. “The very first act of copulation was done sideways, square-dance style.”

Antiarchs are primitive forms of jawed fish called placoderms that lived in lakes more than 380-million years ago. They are known from fossils, only a few centimetres long, dug up in China, Estonia and Orkney in Scotland.

Like so many in science, the discovery came about by chance. Last year, Long was working in a palaeontologist’s laboratory in Tallinn, Estonia, when he was handed a box of placoderm bones. Among them he found a plate with a strange, grooved bone attached. 

He had studied placoderms all his life but was at a loss to explain what it was. Later that day, the penny dropped: “It was a clasper, a sex organ, and it was the oldest and the most primitive one yet found on the planet,” he said.

The finding prompted a search for other samples that led to private collections of fossils in the United Kingdom and the Netherlands. From those they gathered more evidence of male antiarchs with their claspers still attached.

Further examination of the antiarchs revealed the first evidence of discrete female sexual organs, in the form of small genital plates in exactly the right position to facilitate sex.

The studies led Long and his colleagues to solve another mystery that has puzzled specialists for more than a century — the purpose of antiarchs’ arms. “Now we know that if your sexual organs are rigid and fixed to your whole body, then little arms are very useful for helping to link the male and female together,” Long said.

“The male can get his large L-shaped sexual organ into the right position to dock with the female genital plates, which are like cheese graters — very rough — so they act like Velcro, locking the male organ into position to transfer sperm.”

“This is the very earliest act of copulation that we know of,” Long said. Details of the study are reported in the journal Nature. — © Guardian News & Media 2014

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