Australian scientists have found sexual intercourse was pioneered by ancient fish about 385 million years ago in Scotland.

In what is being described as one of the biggest discoveries in the evolutionary history of sexual reproduction, the scientists found male fossils of the Microbrachius dicki, which belong to a placoderm group, developed bony L-shaped genital limbs called claspers to transfer sperm to females.

Females, for their part, developed small paired bones to lock the male organs in place for mating.

Placoderms are the earliest vertebrate ancestors of humans, said South Australian palaeontologist Dr John Long, who led the research.

"Placoderms were once thought to be a dead-end group with no live relatives, but recent studies show that our own evolution is deeply rooted in placoderms and that many of the features we have - such as jaws, teeth and paired limbs - first originated with this group of fishes," Dr Long said.

This new finding, he added, showed "they gave us the intimate act of sexual intercourse as well".

Palaeobiologist from Britain's Oxford University Matt Friedman described the findings as "nothing short of remarkable" and said they suggested much more could be learned from the fossilised fish.

Flinders University's Dr Long, whose study was published in the journal Nature, discovered the ancient fish's mating abilities when he stumbled across a single fossil bone in the collections of the University of Technology in Tallinn, Estonia, last year.

Australian scientists have found that the ancient fish pioneered sexual intercourse around 385 million years ago. ( Flinders University )

The research then involved an international team of scientists who analysed fossil specimens from museum collections across the world.

The specimens demonstrate the first use of internal fertilisation and copulation as a reproductive strategy known in the fossil record.

Measuring about eight centimetres in length, Microbrachius lived in ancient lake habitats in Scotland, as well as parts of Estonia and China.

Dr Long explained "Microbrachius" meant little arms, but said scientists have been baffled for centuries by what these bony paired arms were actually there for.

"We've solved this great mystery," he said.

"They were there for mating, so that the male could position his claspers into the female genital area."

In one of the more bizarre findings of the study, Dr Long said the fish probably copulated from a sideways position with their bony jointed arms locked together - making them look more as if they were square dancing than having sex.

"The only way it would have worked was if they were sitting side-by-side," he said.

"These L-shaped claspers on the male would reach to the centre of the female, where she had two little genital plates which were pretty rough, a bit like cheese graters, so that they could lock the male clasper into position like velcro.

"This enabled the males to manoeuvre their genital organs into the right position for mating."

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