Virtually everything we think we know about dinosaurs is conjecture based on their remains. That doesn't stop us from enjoying the Jurassic Park franchise. We consider their representations in those movies to be functional and believable. If a druid took the form of a brown bear after examining a stuffed one, I'd like to think he could guesstimate enough to functionally depict one. Maybe not pass as one among the real thing because he won't know behavior. But I don't think that's generally why a druid would turn into a grizzly anyway.
Also, all this is contingent on how much one like to apply the ol', "cuz magic."
Also, all this is contingent on how much one like to apply the ol', "cuz magic."