Copulation was invented by ancient fish
Among the unlikely cold sandstones of Scotland, a 19th century naturalist, Robert Dick, discovered fossils of tiny freshwater fish that were among the very first finned animals. These armoured placoderms were weighed down by a new invention bony plates, which kept them safe from predators. Bothriolepis is the most familiar but Mr. Dicks related specimen had strange and bony, elongate limb-like fins, so it became known as Microbrachia.
390 million years ago, arms were not evolved. These limbs were not only used as arms, it seems. Before they disappeared with the advent of further fishy inventions, Microbrachia dicki and other placoderms were using these long appendages for the male to clasp his mate, then transmit his sperm into her. This was the first copulation, the first internal fertilisation, and it has been popular ever since.
John Long and Elga Mark Kurik joined with numerous colleagues from Flinders University, the Universities of Adelaide and Amsterdam (Netherlands), the ANU in Canberra, Tallinn University of Technology, and museums ranging from Los Angeles and South Australia to the Natural History Museum in London. Together, they produced the paper responsible, - Copulation in antiarch placoderms and the origin of gnathostome internal fertilization in yesterdays Nature.
This progress shown by the jawed vertebrates illustrates the early importance of internal fertilisation and the possibility that live-bearers (a little like mammals) were evolving. Now we have to decide if all of the external fertilisation found in the frogs, as well as most modern bony fish, evolved from these internally fertilised fish. It does seem a little backward if this is shown to be true. Because these early 8cm (3 inch) fish lived in a freshwater lake or similar, the chances of external fertilisation could have been reduced. More specimens of related placoderms have been found in Chinese Middle Devonian sandstones, so perhaps this story will run a little longer.The earliest bony fish alive are the much-threatened sturgeons.
Professor Long first discovered an embryonic placoderm inside its mother in 2008. His own words imply his delight with the earliest vertebrates: it dawned on me after studying the specimen that this was the earliest evidence of vertebrates having sex by copulation not just spawning in water, but sex that was fun. Next came his discovery of this boned sex organ of Microbrachius in Estonia, then finally the comparison of a mature pair. This led to the discovery of bony plates on the female which could lock the males arms around her. The pair would have had to perform a square dance-like movement to manoeuvre the male into his position. Then, with a sideways stance, the couple could perform the age-old, dance of procreation!