Swedish researcher finds missing piece of fossil puzzle

That is the conclusion drawn by Professor Per Erik Ahlberg, Uppsala University, from his discovery of a so-called “clasper” in a primitive fossil fish earlier this year. The research results are published today in Nature.

In February this year, a paper published in Nature by a team of Australian and British researchers showed that placoderms, a group of ancient fishes that died out more than 350 million years ago, gave birth to live young. Beautifully preserved fossil embryos in the body cavity of the placoderm Incisoscutum showed that these fishes, close to the common origin of all jawed vertebrates, had a mode of reproduction similar to modern sharks.

Live birth requires internal fertilisation; sharks achieve this by using a “clasper”, an extension of the pelvic fin that functions like a penis. The authors looked for a clasper in their placoderm fossils but couldn't find one, so they were forced to argue that it had been made of soft cartilage and had not been preserved.

Shortly afterwards, Per Erik Ahlberg from Uppsala University visited one of the Australian researchers and spotted a perfectly preserved bony clasper in one of their Incisoscutum fossils.

“It was lying in plain view but had been misinterpreted as part of the pelvis and overlooked,” he says.

Together with the original authors he is publishing a short paper in this week's Nature that presents this missing piece of the puzzle and completes the picture of placoderm reproduction from mating to birth.

“It provides a pedigree of nearly 400 million years for the “advanced” and seemingly specialised reproductive biology of modern sharks,” says Per Ahlberg.