Dinosaurs, a diverse group of reptiles from the clade Dinosauria, first appeared in the Triassic period and became dominant terrestrial vertebrates following the Triassic–Jurassic extinction event. They are categorized into avian dinosaurs (birds) and extinct non-avian dinosaurs, with over 900 genera and 1,000 species identified through fossils. Notable dinosaurs like Tyrannosaurus rex, Triceratops, and Velociraptor are famous both in scientific communities and popular culture, showcasing their significant roles in Earth's history.