Dinosaurs were a diverse group of reptiles that lived during the Mesozoic Era, which lasted from about 252 to 66 million years ago. They were the dominant terrestrial animals for about 135 million years, and many different species of dinosaurs existed during this time. Dinosaurs were divided into two main groups: the Saurischian, which included theropod dinosaurs like the T. rex and sauropod dinosaurs like the Brachiosaurus; and the Ornithischians, which included herbivorous dinosaurs like the Stegosaurus and Triceratops. Dinosaurs were wiped out around 65.5 million years ago during the Cretaceous-Paleogene extinction event, which was likely caused by a massive asteroid impact. Despite this mass extinction, birds, which are considered living dinosaurs, survived and continue to thrive today.
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