Fossils rewrite the history of sex

Scientists in Adelaide have discovered the origins of sexual intercourse, through studying fossils of armour-plated fish called antiarch placoderms, that became extinct some 360 million years ago.

Palaeontologist John Long and his team say the fish were the first animals to develop specific male and female genitalia.

"This was totally unexpected,” John Long, from Flinders University, told Nature. “Biologists thought that there could not be a reversion back from internal fertilisation to external fertilisation, but we have shown it must have happened this way.”

The boney fish gave rise to all current vertebrates with jaws – including humans.

Before the placoderms all reproduction was through external fertilisation, in which sperm and eggs are expelled into the water to unite. But Long's team found evidence the placoderms changed all this. They discovered structures in the fossils that they interpret as bony "claspers" — male organs that penetrate the female and deliver sperm. They say the placoderms would've had sex side by side.

But the discovery that the animals had penetrative sex is perhaps less surprising than the fact that the bony fishes that follow placoderms in the evolutionary tree, show no evidence for internal fertilisation.

Some of their descendants had to "re-invent" sex organs such as claspers in sharks and rays and mammal penises.

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