Acts of copulation have been traced back to ancient animals that were endowed with such cumbersome sexual organs they are believed to have mated side by side.
Fossilized 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 were a kind of jawed fish called placoderms that lived in lakes. They are known from fossils — believed to be 380 million years old — only a few centimeters 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 initially at a loss to explain what it was.
Later that day, he came up with an idea.
“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 UK and the Netherlands. From those Long and his colleagues gathered more evidence of male antiarchs with their claspers still attached. The grooves in the organs are used to transfer sperm from male to female. 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’s team to pose an answer to 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.”
The discovery means sex with internal fertilization evolved early on in the history of vertebrates, but was then lost, with fish reverting back to spawning in water, and then evolved again in a different way, the team said.
“We’ve defined the point in evolution when the origin of internal fertilization in all animals began, and that’s a really big step,” Long said. “In terms of evolution, this is the very earliest act of copulation that we know of.”
Details of the study are reported in the journal Nature.
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