From Academic Kids
The Mesozoic includes three Geologic Periods; from oldest to youngest, the Triassic, Jurassic and Cretaceous. It extended from roughly 251 million years before present to roughly 65 million years. It followed the Paleozoic Era and preceded the Cenozoic Era.
The Mesozoic covers the time when life was dominated by large sophisticated reptiles. The lower (oldest) boundary is set by the Permian extinction. The upper (youngest) boundary is set at the Cretaceous extinction.
Geologically, the Mesozoic starts with almost all the Earth's land collected into a supercontinent called Pangaea. During the Era, Pangaea split into the northern continent Laurasia and the southern continent Gondwana. Laurasia then split into North America and Eurasia. Gondwana broke up progressively into four continents: South America, Africa, Australia and Antarctica.
The Mesozoic is known as the Age of Dinosaurs. It also saw the development of early birds and mammals, and of flowering plants (angiosperms). At the end of the Mesozoic, all the major body plans of modern life were in place although in some cases—notably the mammals—the forms that existed at the end of the Cretaceous were relatively primitive.