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

Even with species alive today we sometime discover that what we thought was 1 species was in fact 2 or that what we thought was 2 species is in fact 1 species

And that is with thing alive today, because nature is a very stubborn thing that refuse to neatly to fit into or human definition

So now you can imagine how complicated it is to do with millenias old bone shaped rock

TL:DR, it’s very difficult and we get it wrong a lot and constantly have to correct our old assumptions when new evidence are found