Mesozoic era is the era during which reptiles were dominant. It includes three periods : Triassic ( 240 million years ago), Jurassic (195 million years ago) and Cretaceous (135 million years ago). Origin of dinosaurs occurred during triassic period. During Jurassic period, lizards, crocodiles and alligators originated. Dinosaurs became large and reptiles were dominant during this period. During cretaceous period, dinosaurs got extinct.