Georges Cuvier was one of the first scientists to realise that the earth had once been full of enormous reptiles.
Before that, people had thought that the only animals to have lived on earth are the ones that are alive now.
He got to know the skeletons of many animals really well, so when he saw a new skeleton, he knew whihc ones it was most like.
We watched the first few minutes of this film, Sea Rex, to see how one fossil, changed the way we see the history of our planet. You might like to see more of it: