The mode of reproduction seen in modern sharks is nearly 400 million years old, according to research conducted by Uppsala University researcher Per Erik Ahlberg, 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 unsuccessfully searched for a clasper in their placoderm fossils and, as a result, concluded that the pelvic extension had been made of soft cartilage and had not been preserved.
Shortly thereafter, Per Erik Ahlberg from Uppsala University join one of the Australian researchers and indetified a perfectly preserved bony clasper in one of the Incisoscutum fossils.
"It was lying in plain view but had been misinterpreted as part of the pelvis and overlooked," he says.
Together with the earlier research, Per Ahlberg's work 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.
Source: Uppsala University