When dinosaurs first showed up on Earth around 230 million years ago, they were nobody important. The world of the Late Triassic was ruled by other reptile groups: bulky, armored beasts called rauisuchians, and rhino-like dicynodonts that lumbered across the land.
The earliest dinosaurs were small - typically about 3 feet long, lightly built, running on two long back legs with small grasping arms. Animals like Eoraptor and Herrerasaurus were fast, nimble, and probably ate small prey. They lived in the shadows of bigger animals, easy to overlook.
Then around 200 million years ago, a mass extinction wiped out most of those other reptile groups. Dinosaurs survived - and suddenly had the world to themselves. From that moment on, they grew, diversified, and dominated for 135 million more years.
