The earliest known english-language account of a dinosaur bone is in Robert Plot’s 1677 The natural history of Oxford-shire: being an essay toward the natural history of England. This is how he explains what we now know as a megalosaurus thighbone:

There happily came to Oxford while I was writing of this, a living Elephant to be shown publickly at the ACT, An. 1676, with whose Bones . I compared ours; and found those of the Elephant not only of a different Shape, but also incomparably different to ours, though the Beast were very young and not half grown. If then they are neither the Bones of Horses, Oxen, nor Elephants, as I am strongly persuaded they are not . It remains, that (notwithstanding their extravagant Magnitude) they must have been the bones of Men or Women: Nor doth any thing hinder but they may have been so, provided it be clearly made out, that there have been Men and Women of proportionable Stature in all Ages of the World, down even to our own Days

The whole account is pretty interesting. He speculates about the origin of the bones and their medicinal uses and discusses the particulars of a few giants.