Study author and palaeontologist Dr Kate Trinajstic, of Curtin University in Perth, says the clasper was discovered in a fish specimen uncovered in the Gogo region of Western Australia in 2001.
She says the team originally discounted the bone as the reproductive organ because they thought it was part of the pelvic gurdle.
On closer inspection, Trinajstic says they realised it was a sexual organ.
"We were surprised because it's so big," she says. "We were expecting something smaller."
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(image credit: John Long)