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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The way a group of organisms is defined is all descendants of a common ancestor. It’s a little weird if you’re not used to thinking about it that way, but it makes sense in biological definitions.
Historically, “dinosaur” was a colloquial way to group Iguanodon, Megalosaurus, and Hylaeosaurus into a big group of BIG ANCIENT REPTILES. This was in the 1800’s when science was a little less scrutinized than it is today. As more and more species were discovered over the past ~200 years, and scientific understanding of evolution became better understood, it became clear that more defined classification must be established.
Thus, it was decided that the modern definition of “dinosaur” is the common ancestor of all modern birds (more specifically, passeridae) and Triceratops, and all its descendants. This encompasses the same animals as the classical definition – it’s just more scientifically defined. There is a common ancestor between Triceratops and passerines, and that, and its descendants is the definition of “dinosaur.”
Pterosaurs and ancient aquatic reptiles share a common ancestor with the dinosaurs, of course, but their lineages split off before this clade.
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