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Dinosaurs are a diverse group of mostly land-based animals that are related to today's reptiles and birds.  In fact, while dinosaurs are commonly seen as extinct and known only by their fossils, paleontologists technically include modern birds as a surviving group of avian dinosaurs.  Both avian and non-avian dinosaurs evolved from a more ancient group of reptiles roughly 240 million years ago, diversifying in shape and size before mostly dying out in a mass extinction event around 65 million years ago.