The ancient armored fish called antiarch placoderms started the evolution of sexual reproduction about 385 million years ago, researchers said.

A group of scientists from Flinders University in Australia discovered the origin of sex by studying ancient fish fossils named Microbrachius dicki.

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M dicki, which measures about 8 centimeters (3 inches) in length, lived in ancient lake habitats in Scotland, Estonia and China.

Australian paleontologist John Long along with other researchers found that male fish fossils of M dicki are the first-known animal that is believed to have engage in sexual intercourse.

According to the recent discovery, the M dicki, which belongs to a placoderm group, had bony L-shaped genital limbs called claspers. These primitive creatures used their claspers to transfer sperm to the female's tiny paired genital plates.

The primitive bony fish were the first animals to develop specific male and female genitalia and also the pioneers in using internal fertilization to reproduce, reports said.

In the latest discovery, the odd shapes and bony structures of the ancient fish implied a different way of mating.

Professor Long said the only way possible M dicki could copulate was through "sideways, square dance style, with their little arms entangled."

The M dicki fossils discovery is not just about bony structures, "it's about the evolution of behavior, of when sex first became fun," Professor Long added.

The new findings came after Long examined a single fossil bone in the collections of ancient fish fossils stored at University of Technology in Tallinn, Estonia in 2013.

TagsAncient fish, Microbrachius dicki, evolution of sexual reproduction