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?
In: 103
Dinosaurs are not lizards at all, lizards are a completely different branch of the reptile family tree.
Dinosaurs are a biological *clade,* meaning all of the species descended from a particular common ancestor. Typically, the dinosaur clade is described as everything descending from the most recent common ancestor of Triceratops and modern birds and are distinguished by the structure of their hips, ankles, and some other bones. The first dinosaur, i.e. the most recent common ancestor of all dinosaurs, is estimated to have lived around 230 million years ago. Other than birds (which *are* dinosaurs) the closest living relatives to dinosaurs are the crocodiles; both dinosaurs and crocodiles belong to a group of reptiles called “archosaurs.”
Things that are not dinosaurs:
1. Dimetrodon: Dimetrodon went extinct 40 million years before the first dinosaurs evolved, and are therefore excluded based on the above definition. Moreover, dimetrodon is not even particularly closely related to dinosaurs and it is a stretch to even call it a “reptile;” it’s actually much more closely related to mammals than any living reptile. Their (and our) last common ancestor with dinosaurs would have lived around 320 million years ago.
2. Pterosaurs: Pterosaurs are in fact archosaurs, part of the same group as dinosaurs and crocodiles, but split off shortly before dinosaurs evolved making them one of the dinosaurs’ closest relatives. They lived alongside dinosaurs and went extinct around the same time. Although they could fly, they evolved flight completely independently from birds, and were physiologically more like bats, with membranes of skin stretched over their fingers.
3. Marine reptiles, i.e. the ichthyosaurs, plesiosaurs, mosasaurs, etc: Where some of these groups belong on the reptile family tree is somewhat unclear, but they are not dinosaurs. Ichthyosaurs are very mysterious in their origins, but fossil evidence shows they definitely evolved before dinosaurs. Plesiosaurs are also difficult to classify, but the modern consensus is that they are related to turtles. Mosasaurs are unambiguously lizards, and are closely related to snakes and monitor lizards. Generally speaking, there were no marine dinosaurs (although some, like Spinosaurus, may have been semi-aquatic).
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