>> “Where did we find out what they looked like?” <<
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Years and years of painstaking digging with spoons and tooth brushes, at numerous different sites, because until recently most specimens were 10 - 20% of the animal at best.
You have this romantic image of digging up a complete specimen intact that gave a clear image of the living animal, and that is pure bunk.
Where did we find those fossils?
Could the civilization that built Angkor Watt have used descriptions they got handed down through their history? Maybe from previous civilizations?
Is it possible that the reason we don't find complete specimens is that all the easy to find, complete specimens were found 1000's or more years ago?
Is it possible that there were still some Stegosaurus's alive approx. 1000 years ago? (I would say it is possible).
Is it possible there were other 'dinos' that didn't die off completely, and survived much longer than we think?
You said it yourself. Most of our 'depictions' of 'dinosaurs' are based on a small amount of fossils, and yet we believe we have a clear 'image' of what they looked like. Scientists claim to provide a complete 'depiction' even after finding ONE BONE.
What makes you think the depiction the builders of Angkor Watt or that we have are actually correct? Have you seen a real Stegosaurus?