Final answer:
The Mesozoic Era, also known as the Age of Dinosaurs, is considered significant in geological history due to the extinction of reptiles and the appearance of mammals.
Step-by-step explanation:
The era characterized by the extinction of reptiles and the appearance of mammals is the Mesozoic Era. This era, also known as the Age of Dinosaurs, lasted from 245 to 65 million years ago. It is significant in geological history because it marked the dominance of dinosaurs and the subsequent extinction that opened up new ecological niches for mammals to evolve and thrive.