225 million years ago: earliest dinosaurs.
Some examples of the earliest dinosaurs are the fossils of Herrerasaurus and Eoraptor. Many come from Argentina, from a region which has produced many other fossils of different species.
Dinosaurs evolved from reptiles called archosaurs 225 million years ago. Their main characteristic was to be bipedal. At that time, all continents were united forming the supercontinent Pangea, which had been the result of the collision between Laurentia and Gondwana 300 million years ago.
The success of the dinosaurs was because they evolved soon after the Permian mass extinction, 251 million years ago when 70% of species on land went extinct. Dinosaurs were able to fill many of the ecological niches left empty by the catastrophe.
Dinosaurs came to dominate the whole planet. There were herbivores and carnivores; terrestrial, marine, and flying gliders; covered with fur or with feathers; small, medium, large and gigantic.
They became extinct 65 million years ago when a meteorite fell in what is now the Yucatan Peninsula. The effect was extensive climate changes after which 65% of species disappeared. The consequences of the disaster were many. Among others, there was darkness, vast amounts of toxic dust in the atmosphere and a decrease in temperature.
It is from the only surviving dinosaurs that modern birds descended. Thomas Henry Huxley, the great friend, and supporter of Darwin, was the first to suggest this idea.