which era characterized by the extinction of reptiles and the appearance of mammals is considered significant in geological history

Mesozoic Era

Explanation:

The "Mesozoic Era" is divided into three periods: Triassic, Jurassic and Cretaceous. During the last period (Cretaceous), which was considered the longest, the extinction of reptiles occurred and the new mammals appeared. This event was also known as the "Cretaceous-Paleogene Extinction Event." Around 3/4 of plants and animals died. Such event became significant boundary that laid between the Cenozoic Era and the Mesozoic Era. This made it significant in geologic history.