Origin of mammals:  Mammals appeared on earth in the Jurassic period, approximately 195 millions years ago. Mammals, birds and snakes are considered to be originated from the common ancestor, cotylosaurs or stem reptiles (Figure 10.2). The initial mammals, tree shrew lived for several million years along with giant reptiles dinosaurs. The extraordinary ability of mammals to the changed environment, active life due to warm body and sensitivity towards environment. Post extinction of dinosaurs, mammals evolved into a variety of forms to capture the available space (water, land and air) on earth.  

Figure 10.2 : Origin of mammals, birds and reptiles.