There remains some question which dinosaur was actually the earliest of its kind. The oldest claimant is of the order Saurischia (literally ‘lizard-hipped’ dinosaurs), the Nyasasaurus, dated at 243 million years ago. The next oldest saurischians are dated between 232 and 230 millions years ago.
The earliest true dinosaurs of the Ornithischia order (the other major order of the Dinosauria clade) are likewise dated at 230 to 220 million years ago, the oldest of which is the Pisanosaurus.
As mentioned in:
History of the World (Part 1) — The Damned