Dinosaurs were egg layers and are distinguished from all animals by their small brains, having brains weighing less than one pound to control bodies later weighing as much as 40 tons. 
Rapidly evolving reptilian dinosaurs soon became the monarchs of this age. They appear 120 million years ago. The largest of the dinosaurs originated in western North America. Land-animal life reached its greatest development, in point of size, and had virtually perished from the face of the earth by the end of this age.  Largest dinosaurs lived in Rocky Mountains. 
60 million years ago, though the land reptiles were on the decline, the dinosaurs continued as monarchs of the land, the lead now being taken by the more agile and active types of the smaller leaping kangaroo varieties of the carnivorous dinosaurs.  None ended buried in Australia. 
Dinosaurs were short-lived species that soon became extinct and suffer destruction, because of having too little brain substance in comparison with body size. 
These massive creatures became less active and strong as they grew larger and larger; but they required such an enormous amount of food and the land was so overrun by them that they literally starved to death and became extinct—they lacked the intelligence to cope with the situation. 
Flying pterosaurs were not the ancestors of the true birds of subsequent ages. They evolved from the hollow-boned leaping dinosaurs, and their wings were of batlike. They represent the nonsurviving strains of bird ancestry. 
It was from an agile little reptilian dinosaur of carnivorous habits but having a comparatively large brain that the placental mammals suddenly sprang.