Mesozoic era is known as "era of reptiles" as well at the "age of creepers" because evolution of reptiles, development, dominance of reptiles and finally extinction of some reptiles occurred within Mesozoic era.
Mesozoic era is called the age of reptiles because this is the era when dinosaurs dominated the earth. In this period life diversified rapidly and giant reptiles like dinosaurs and other beasts were on land. This era spanned from 252 million years ago to about 66 million years ago and this is also the era where the mass extinction of dinosaurs happened.