How do scientists know that a fossil fragment they found belongs to an already existing dinosaur and not a new species?

Question

Say when a scientist discovers a new skull fragment from a T-Rex, how do they know it’s a skull fragment from a T-Rex and not like a fragment of some unknown species of dinosaur?

In: Biology

Answer 1

Question on top of your question, how do they know it’s a dinosaur bone in the first place? Why not a new species of extinct mammals?