[shagie]

This is a confusion about what a "dinosaur" is. It is a particular branch of reptiles, not just an ancient one. The first dinosaurs were in the Triassic (roughly 240 mya). It was likely the Permian-Triassic extinction that opened the evolutionary niches for dinosaurs to evolve into (much like the Cretaceous extension event opened up the way for the mammals).

While there were creatures that looked like dinosaurs in the Permian period (298 mya) - creatures like the dimetrodon (the one with the sail on its back that you'd find next to dinosaurs in the museum - though the dimetrodon is of the branch that lead to mammals rather than reptiles) and early lizards and turtles - these aren't dinosaurs.