What 2 extinction shape the evolution of dinosaurs

Permian - Triassic

Cretaceous - Tertiary

Explanation:

There are two extinction events that influenced greatly the evolution of dinosaurs. The first one was the Permian -Triassic mass extinction, where 95% of the species went extinct. This mass extinction ended the reign of the sinapsids, thus it opened up space for the reptilians, including the ancestors of the dinosaurs. Once the majority of ecological niches were free for taking, the dinosaur ancestors quickly started to evolve and soon after gave rise to the first true dinosaurs. As they were the ones that managed to exploit the environment the fastest, they became the dominant animals on the planet. That lasted until the Cretaceous -Tertiary mass extinction. The dinosaurs died out during this extinction, so their reign was over. Only one branch of the dinosaurs lineage managed to survive, the birds. The birds initially managed to dominate around the world, but with so many empty niches the mammals quickly evolved and managed to out-compete them.