I’ve gone my entire life being told “that’s not a dinosaur that’s a (insert what it is) but why? What’s makes one giant lizard a Dinosaur but a different giant reptile not a dinosaur?
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Modern taxonomy generally uses phylogenetics which organizes organisms based upon common ancestors. Older taxonomy methods used physical characteristics to organize them. Both methods have some deficiencies. There can be some cross “pollination” between groups which means that some branches can connect to multiple locations. Classifying based solely upon the characteristics can be problematic when unrelated species develop similar adaptations to fill the same general niche (convergent evolution). One example (which didn’t necessarily fool taxonomists) are hyenas. They are quite doglike in appearance, but are actually much more closely related to cats.
One of the major descriptive characteristics of dinosaurs vs other reptiles is the erect nature of the hip/leg joint. Reptiles tend to have a sprawled nature with the legs coming out more or less horizontally before potentially bending down at the elbow/knee–think of an crocodile or turtle. Dinosaurs legs tended to go straight down from the body–think of a tyrannosaur or bird.
Considering how old the fossils of dinosaur are we can’t run DNA tests, and soft tissues don’t fossilize well. Most classification are therefore based upon bone morphologies and other evidence like where they were found. It is strongly suspected that at least some (if not all) dinosaurs were warm blooded as opposed to cold blooded like other reptiles (after all their descendants (birds) are). In 2000 they thought they may have found a fossilized 4-chamber heart in one specimen, however, further analysis has cast doubt on it being a heart, so the mystery remains (birds have a 4-chamber heart while reptiles have only 3).
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