There is no single "oldest dinosaur" because the fossil record is constantly being updated with new discoveries. However, there are a few contenders for the title of oldest known dinosaur:

* Nyasasaurus parringtoni: Discovered in 1956 and described in 2012, this dinosaur lived around 243 million years ago during the Triassic period. While its remains are fragmentary, they are considered to be among the earliest known dinosaur fossils.

* Eoraptor lunensis: Discovered in 1991, this small dinosaur lived around 231 million years ago. Its skeletal structure suggests it may be closely related to the common ancestor of all dinosaurs.

* Herrerasaurus: Discovered in 1963, this dinosaur lived around 231 million years ago during the Late Triassic. It was a predatory dinosaur with a longer skull and a more robust build than Eoraptor.

These are just a few examples, and the understanding of dinosaur evolution is constantly evolving. As more fossils are discovered, we may find evidence of even earlier dinosaurs.