We don’t, every year we discover more and the picture gets better and better. That said there’s a lot of what we see that sre just best guesses. Like did most dinosaurs actually have feathers and sound like a duck. We are always learning more as we catalog more fossils or even discover new information from old fossils using new techniques.
Things like size, shape are the easier parts. Things like skin color, behavior and lifestyle are harder. Some dinos we only have a couple pieces of and some we have lots and lots of examples.
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