The early ancestors of dinosaurs form a group collectively known as dinosauromorphs, and in many ways they resembled their descendants almost to a tee. They were all small, slender running animals with generalized diets that lived in the shadow of larger, more powerful creatures. It has been assumed that the first dinosaurs emerged about 231 million years ago, at the start of the Late Triassic.
However, new evidence from Argentina has pushed the date further back than we had ever thought. We now know that dinosauromorph diversification and evolution was very fast during this time. The Chañares Formation of Argentina might be considered the cradle of dinosaur evolution for the simple reason that so many of their ancestral cousins existed here. Plus, some of the very earliest dinosaurs and dinosauromorphs are almost 10 million years older than the Chañares genera. By any standards this is a very rapid evolution.
The rocks of the Chañares date back to at least 236 to 234 million years ago, the Ladinian Stage of the Middle Triassic. But this is due to newer findings. The scientists who dated the beds had put them as a Late Triassic fossil site instead. These beds were a good 10 million years older than once thought.