Scientists found evidence that an armored fish known to them for almost 130 years might have pioneered sex using internal fertilization.

Microbrachius dicki, which was first discovered in Scotland's freshwater, has an average length of 3 centimeters. Scientists believe that the species lived about 385 million years ago.

Paleontologist John Long and his colleagues from Finders University in Australia made the discovery when they observed a clasper attached to fossils of the M. dicki. The researchers decided to look at other fossils in England and the Netherlands. They were able to find male claspers and female genital plates. The bone structure of the females suggested that it could allow penetration of the male clasper to pass on sperm.

"The male has large bony claspers. These are the grooves that they use to transfer sperm into the female. They couldn't have done it in a 'missionary position'," said Prof. Long to BBC News. "The very first act of copulation was done sideways, square-dance style."

The discovery supported a 2008 finding, in which a 380-million-year-old Microbrachius dubbed as the Materpiscis, or "mother fish" was found to have embryos inside the body. The discovery implied that the fish underwent internal fertilization, but researchers couldn't determine how exactly this was done.

"Microbrachius has been known to science for nearly 130 years. How did we overlook this?" said Martin Brazeau, a palaeontologist from the Imperial College London to National Geographic. "We've been assuming the evidence simply isn't there, but it's been right under our noses for a long time."

Scientists presumed that the M. dicki is the earliest evidence of sexual differences in the fossil record, and it might be the origin of internal fertilization among animals. Prior to this discovery, fish were believed to reproduce only by spawning in which eggs and sperms were released by male and female fish to fertilize.

Details of the finding were published in the Oct. 19 issue of Nature.