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What is the sine qua non of dinosaurs? I have searched several books.

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If I recall my comparative vertebrate anatomy correctly, it is the structure of the pelvis (and maybe some skull characteristics too?). But there is a very easy way to explain it to a 5yo: if it swims, flies, or walks like a lizard, it's not a dinosaur. (Walks like a lizard means legs jut out to the side, whereas with dinosaurs, as with humans, they are directly beneath the body.) Ancient aquatic reptiles and flying reptiles are often confused with dinosaurs, but are actually not.

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According to my paleontologist-to-be-ds14, the main thing that separates dinosaurs from other reptiles it the fact that the top of their thigh bone is bent inward, so that, although the hip joint faces outward, the leg extends downward. With reptiles, the legs sprawl out to the sides because their thigh bone does not have the bend at the top.

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