Geologists divide time into eras and periods. Dinosaurs lived in the Mesozoic Era, which lasted from about 252 to 66 million years ago (written Ma, “mega-annum”), roughly 186 million years in all. It has three periods:

• Triassic: about 252 to 201 Ma
• Jurassic: about 201 to 145 Ma
• Cretaceous: about 145 to 66 Ma

The first dinosaurs, such as Eoraptor and Herrerasaurus from Argentina, were small and appeared about 230 Ma in the Triassic. The non-bird dinosaurs died out at the end of the Cretaceous, about 66 Ma.