If we look at the skeletons of contemporary lizards, they’re all rather similar. Likewise fish. Yet both these groups have wildly different externals as to color, textures, etc.
Likely the case with truly ancient organisms. Someone posted a picture of a hippopotamus skull and how it might be “fleshed out” by a researcher who’d never actually seen one of these critters…. Not much resemblance.

But we can certainly get the basic body shape quite closely.