Claspers are modifications of the pelvic fins that form the penis-like organ found in sharks. These organs are used to deposit sperms into the genital duct of sexually receptive females.

It is also possible to consider the mating habits of modern-day sharks to understand how ancient placoderms reproduced.

When most species of shark mate, the male holds the female in position by biting onto her fins, before arching the body so that the pelvic fins are brought close to the mating partner.

'This is quite an advanced mating behaviour. It gives the first evidence of complex foreplay,' Trinajstic said.

For researchers, this sort of finding has broad implications in the study of the evolution of complex vertebrates, including humans.

These findings were published in Nature.