An extinct fish with the intriguing scientific name "Microbrachius dicki" has been identified as the first animal to have sex.

Microbrachius dicki was some eight centimeters long and lived 385 million years ago in lakes in what is now Scotland. Fish today normally reproduce by spawning but Microbrachius dicki did it the way mammals do it, said research published in the journal Nature.

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An international team of researchers claims Microbrachius dicki is the first-known animal to stop reproducing by spawning and instead mated by having sex.

In their investigation, researchers noticed one of the M. dicki specimens had an odd L-shaped appendage. More investigation later revealed this was the male fish's genitals.

"The male has large bony claspers. These are the grooves that they use to transfer sperm into the female," said study lead author Prof. John Long from Flinders University in Australia.

The fish's little arms served to link the male and female together. Once linked, the male positioned his large L-shaped sexual organ to dock with the female's genital plates. The female's rough plates acted like Velcro, locking the male organ into position to transfer sperm.

Because of its anatomy, M. dicki probably mated side-by-side. Prof. Long said M. dicki couldn't have sex in a missionary position.

"The very first act of copulation was done sideways, square-dance style."

The fish were able to stay in position during sex with the help of their small arm-like fins. Despite M. dicki fossils being common, scientists didn't notice the sexual organs until now.

"We have defined the very point in evolution where the origin of internal fertilization in all animals began," said Prof. Long.

"That is a really big step."

Fish, however, went back to spawning as they evolved. Spawning is a more efficient method of reproduction instead of one-on-one mating.

Scientists said it took millions of years for copulation to make a come-back in the ancestors of sharks and rays.