Mesozoic Era is the devolvement of life forms of increasing complexity including reptiles, dinosaurs, small mammals birds conifers, cycads and flowering plants from 248 million through 65 million years ago. The end of the Mesozoic Era is mark by a mass extinction of the dinosaurs, leaving only their fossilized remains to tell their story. Mesozoic means " middle life" it's divided into three major periods the Triassic, Jurassic, and Cretaceous. The climate of the Mesozoic was varied alternating between warming and cooling periods. Overall however, the Earth is hotter than it is today.
" ... Because the fossil record is incomplete, it's difficult to say exactly what caused the extinction" Extinctions, Mesozoic Era; Age of the dinosaur www.livescience.com