#prehisoric

Currently, that would be Staurikosaurus!

Staurikosauus is from the Lower Santa Maria Formation, and has been dated to 233.23 million years old, making in about 1.5 million years older than other very early dinosaurs like Eoraptor and Herrerasaurus from the Ischigualasto Formation!

However, if we want to delve into questionable dinosaurs, there's a genus named Nyasasaurus that is known from very fragmentary remains, and may or may not be a dinosaur!

Nyasasaurus is from the Manda Formation in Tanzania, which seems to have been roughly 235-240 million years old, although dating is a bit shaky with this one. If Nyasasaurus is really a dinosaur it would be the oldest known, but again we don't have enough of the skeleton to be sure! The Wikipedia reconstruction of it literally looks like this, to give you an idea of how uncertain its anatomy is:

Standing an estimated three to four feet tall, Homo luzonensis—classified as a separate species on the basis of 13 bones and teeth from two adults and one child found in a cave on the island of Luzon in the Philippines—adds another twist to the increasingly complex timeline of human evolution. Some 50,000 to 67,000 years ago, the diminutive hominin roamed Earth alongside a host of other early humans, including Neanderthals, Denisovans, similarly petite Homo floresiensis, and our own species, Homo sapiens.