380-million-year-old placoderm fish, Incisoscutum, from Gogo, Western Australia, showing an embryo inside it. This gave clues that these fishes must have been reproducing using internal fertilisation.
Image: Natural History Museum, London
Source: P50934 from the Natural History Museum, London
Museum Victoria’s research on the 380-million-year-old fossil fishes from Gogo, Western Australia, has resulted in some big discoveries in the past three years.
Firstly, we uncovered the Gogonasus, which showed the intermediate stage of fishes leading to land animals. In May 2008, we unveiled the mother fish, Materpiscis, an armoured placoderm fish with an unborn embryo and fossilised umbilical cord.
Today, Dr John Long, Head of Sciences, announced that his team had discovered how these early fishes were reproducing.
“The new discovery of embryos in placoderm fishes belonging to the arthrodire group was a complete surprise to me," Dr Long said. “We weren’t expecting them, as these fish had showed no visible signs of sexual dimorphism – where males and females have differing external body forms. Then we started looking closely at the rear paired fins on the fish Incisoscutum and found new structures not previously seen by other scientists. These features turned out to be same the structures they used for copulation.”
The pelvic fins in fishes eventually evolved into the legs of modern land animals. They share a similar bone pattern in advanced fossil fishes to that of all animals (femur, tibia, fibula). In ancient placoderms the structure is much more primitive. The new find shows that the pelvic fins in these placoderms bore a long extra lobe, probably used for transferring sperm from the males to the females.
Further clues came from re-examination of other placoderm fishes from the Mount Howitt site in central Victoria. One fish, Austrophyllolepis, showed peculiar, long structures attached to the pelvic fin, which the research team couldn’t explain previously. The evidence for the embryos then made sense.
These fish had pelvic fins built on the same pattern as those in modern sharks. All modern sharks have internal fertilisation, with males inserting parts of the pelvic fin called ‘claspers’ inside the females for reproducing. The new discovery is that these ancient placoderms had developed this advanced form of mating well before the sharks.
The discovery, published in the prestigious journal Nature, documents the very origins of sexual reproduction using copulation in all back-boned animals (vertebrates).
This research project was funded by Australian Research Council Grant DP0772138 ‘Old Brains, New Data’.
Long, J.A., Trinajstic, K., Johanson, Z. 2009. "Devonain arthrodire embryos and the origins of internal fertilisation in vertebrates." Nature 457.
Long, J.A., Trinajstic, K., Young, G.C. & Senden, T. 2008. "Live Birth in the Devonian." Nature 453, 650-652.
Long, J.A., Young, G.C.,Holland, T., Senden, T.J. & Fitzgerald, E.M.G. 2006. "An exceptional Devonian fish from Australia sheds light on tetrapod origins." Nature 444, 199-202.