Placoderms may have copulated in a similar way to sharks.The Museum of Victoria

Fish swimming 380 million years ago may have engaged in penetrative sex in a manner similar to modern sharks, a new study has revealed.

The finding, discussed in a Nature video, also shows that reproduction by internal fertilization was much more widespread in early jawed vertebrates than was previously believed¹.

John Long of Monash University in Clayton, Australia, and Kate Trinajstic of the University of Western Australia in Perth had previously been part of a team that discovered small plates of armour inside a number of fossilized armour-plated placoderm fishes². The group showed that, rather than being dinner, these smaller plates were actually embryos (See: The oldest pregnant mum).

Now, along with Zerina Johanson of London's Natural History Museum, they show that the pelvic girdles of one species of extinct placoderm — Incisoscutum ritchiei — may have had long, articulated pieces of cartilage attached, which the fish could have used during copulation in a similar way to the claspers that male sharks use to penetrate females.

"We have an expression that humans like to get a leg over," Long says. "But these placoderms actually like to get a leg in."

• References

  1. Long, J. , Trinajstic, K. & Johanson, Z. Nature 457, 1124– 1127 (2009).
  2. Long, J. A. , Trinajstic, K. , Young, G. C. & Senden, T. Nature 453, 650– 652 (2008).