Re: is the jaw a really a fossil or is it a bone?
It WAS a bone, but its structure has been totally replaced by rock over the ages. Since this slowly occurs at the molecular scale, the resulting rock is a perfect cast of the original, so you can treat it as the actual bone for anatomical studies.
Amazingly, a few fossils still contain preserved soft tissues (collagen, possibly blood cells) so some of the original animal is still there! The fossils actually smell bad, like something dead (Hell Creek Formation). Doesn't that blow your mind?
Further weirdness, sometimes fossils are subjected to pressure and deform over time. There's a local T-rex skull that's noticeably flattened side to side and curved to the left.