In which geological period did the first dinosaurs appear?

The first dinosaurs appeared during the Triassic Period, specifically in the Late Triassic. They evolved roughly 230 million years ago, not during the later Jurassic or Cretaceous periods.

The Triassic began about 251.9 million years ago after the devastating Permian–Triassic extinction and ended about 201.4 million years ago. It was the first period of the Mesozoic Era, followed by the Jurassic and then the Cretaceous. Early dinosaurs were initially a relatively minor part of terrestrial ecosystems.

Dinosaurs emerged within Archosauria, the broader reptile group that also includes crocodilians and pterosaurs. Fossils from South America, including early forms such as Eoraptor and Herrerasaurus, show that the earliest dinosaurs were generally small to medium-sized, lightly built animals. They later diversified substantially.

A common misconception is that dinosaurs first appeared in the Jurassic because many famous dinosaurs lived then and popular films use that period as a setting. In reality, dinosaurs became ecologically dominant only after the end-Triassic extinction, during the Jurassic. The Permian preceded their appearance, while the Cretaceous was their final major period.