Mammals first appeared in jurassic about 200 million years ago. They are supposed to evolve from the cynodont reptiles, which branched off from the stem reptiles called cotylosaurs. The early mammal were very small and resembled the present tree shrews which live with fear some reptiles dinosaurs for several years. Having extincted of dinosaurs in post mesozoic era (age of reptile) the early Mammals quickly evolved in a variety of forms and invaded the various ecological niche.