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SYDNEY, Oct. 20 -- Scientists have discovered the origins of sex through the study of ancient fish fossils, Nature magazine reported on Monday.

A team led by Australian palaeontologist John Long, from Flinders University in Adelaide, has discovered the time when sexual intercourse developed as a method of reproduction in 430 million-year-old armoured fishes called antiarch placoderms.

Fossils of these creatures show they were the first animals to develop specific male and female genitalia, allowing them to have internal sex. Before fishes developed sex organs both males and females shed their gametes in open water to fertilise.

Long said the sex organ of male antiarch placoderms was two big L-shaped claspers that were inserted into the female's tiny paired genital plates.

"When you look at the shape of these structures they can't possibly do anything in a missionary position," Long told Fairfax Media.

"The only way possible they can do it is sideways, square dance style, with their little arms entangled."

Long's team focused on newly discovered fossils, which suggested the sex act evolved much earlier than first believed.

"This is not just about bony structures on these fish, it's about the evolution of behaviour, of when sex first became fun," Long said.

"Why would something develop these big bony clasper things and place them inside a female unless they enjoyed the act."