Dinosaurs are a diverse group of reptiles that first appeared during the Triassic period, and were the dominant terrestrial vertebrates for 135 million years, from the beginning of the Jurassic until the end of the Cretaceous, when the Cretaceous–Palaeogene extinction event led to the extinction of most dinosaur groups at the close of the Mesozoic era. The fossil record indicates that birds evolved from theropod dinosaurs during the Jurassic and, consequently, they are considered to be a subgroup of dinosaurs, as well as a group of reptiles.
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