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When did dinosaurs first appear on Earth?

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Illustration showing various dinosaur species drawn to scale and grouped by geologic era: Late Cretaceous, Early Cretaceous, Jurassic, and Triassic. Dinosaurs range from large species like Brachiosaurus and Tyrannosaurus to smaller ones like Velociraptor and Compsognathus, with a human silhouette included for size comparison. Measurement bars at the bottom indicate 1 meter and 3 feet for scale reference. — dinosaur A selection of dinosaurs grouped by the geologic interval in which they lived.

Pinpointing the exact moment dinosaurs first appeared is impossible due to the incomplete nature of the fossil record and the gradual, stepwise process of evolution. However, paleontologists generally trace the origin of primitive dinosaurs to the Late Triassic Period (approximately 237 million to 201.3 million years ago).12

While some researchers consider certain Middle Triassic fossils, such as Nyasasaurus, to be among the oldest known members of Dinosauria, these classifications remain a subject of scientific study. Early dinosaur ancestors, such as the archosaurs known as thecodontians, were present earlier in the Triassic. By the Late Triassic, these groups had evolved into bipedal, swift-moving forms that possessed the specific anatomical features—such as a fully open hip socket and a cnemial crest on the tibia—that define true dinosaurs. These early creatures eventually diversified and rose to dominance during the subsequent Jurassic Period.123