As time passed , her confidence in her knowledge increased , and in 1839 she wrote to the Magazine of Natural History to question their claim that a hooked tooth of the prehistoric shark Hybodus represented a new discovery . She had discovered years ago the existence of fossil sharks with both straight and hooked teeth . As time passed , Anning 's confidence in her knowledge grew , and in 1839 she wrote to the Magazine of Natural History to question the claim made by an article that a recently discovered fossil of the prehistoric shark Hybodus represented a new genus , as she had discovered many years ago the existence of fossil sharks with both straight and hooked teeth .