Dinosaurs are a diverse group of reptiles that first appeared during the Triassic period of Earth's history between 243 and 233 million years ago. The exact origin and timing of dinosaur evolution is the subject of current research. Dinosaurs are represented on every continent by both descendant species of dinosaurs and fossil remains.

After the Triassic–Jurassic extinction event 201 million years ago dinosaurs became the dominant vertebrates and remained dominant through the Jurassic and Cretaceous periods. The fossil record demonstrates that modern birds are closely related to dinosaurs that evolved from earlier theropods during the late Jurassic period. This means modern birds are the only known dinosaur lineage to have survived the Cretaceous–Paleogene extinction event that occurred around 65 million years ago.

Based on evolution dinosaurs can be divided into avian (bird) dinosaurs and all other dinosaurs that are not avian. Paleontologists have identified more than 1,000 different species of dinosaurs.

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