Dinosaur Evolution and Classification

What Dinosaurs Are

Dinosaurs are a diverse clade of archosaurian reptiles that first appeared during the Late Triassic, about 233 million years ago. They became the dominant terrestrial vertebrates after the Triassic–Jurassic extinction event and remained dominant throughout the Jurassic and Cretaceous periods. Dinosaurs included herbivores and carnivores, bipedal and quadrupedal species, and animals ranging from small, bird-sized forms to the enormous long-necked sauropods. Their characteristic features included erect hind limbs, egg-laying reproduction, and varied adaptations such as horns, crests, armor, and spines. Modern birds are the only surviving dinosaur lineage and are classified as avian dinosaurs, while all other dinosaurs are considered non-avian and became extinct during the Cretaceous–Paleogene extinction event about 66 million years ago. Dinosaurs are defined through evolutionary relationships and include groups such as theropods, sauropodomorphs, ornithopods, ceratopsians, stegosaurs, and ankylosaurs. The term “dinosaur,” meaning “terrible” or “fearfully great lizard,” was coined by Sir Richard Owen in 1842.

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Dinosaur Evolutionary Timeline

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