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Nagi
>>208465 >>208490
In a technical sense, dinosaurs are reptiles. But by that same technical sense, so are birds and mammals, in that they all came from a reptilian ancestor (archosaurs on the dinosaur/bird side of things, synapsids on the mammal side).
In basic, common vernacular, however, it's become rather inappropriate to refer to dinosaurs as reptiles, because the more we learn about them, the more distant they get from the standard traits of all other reptiles. After all, you don't really see very many endothermic, upright-walking, herding, feathered lizards or snakes. So nowadays, just like reptiles are reptiles and birds are birds, dinosaurs are...dinosaurs. They're the middle ground between reptiles and birds, but for the sake of everyday speech, they're still so radically different from both that it's really not appropriate to lump them in with one or the other entirely.
As for marine reptiles, the reason they are not dinosaurs is because the various marine reptile families all evolved out of reptilian groups completely unassociated with dinosaurs or even archosaurs. Mosasaurs are actually evolutionary brothers to monitor lizards, while plesiosaurs and pliosaurs make up a separate extinct branch, and ichthyosaurs make up yet another extinct branch.
And just for giggles, pterosaurs are a bit closer to dinosaurs than marine reptiles are, but they branched out of basal archosauria separate from the dinosaurs and crocodylians. Pteranodon is as much of a dinosaur as a salt-water crocodile is.
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