Earliest dinosaur
Santa Maria Formation
Brasil (Santa Maria)

While it is currently not possible to directly date dinosaur fossils from millions of years ago, it is possible to date the rocks in which the fossils are found. Using this method, the oldest-known site from which dinosaurs have so far been recovered is the Santa Maria Formation of Rio Grande do Sul, Brazil, where local zircon crystals have been radiometrically dated at up to c. 233.2 million years old, placing them in the Ladinian age of the Late Triassic period. Notable examples excavated in the Santa Maria Formation to date are: Saturnalia tupiniquim, Nhandumirim waldsangae, Buriolestes schultzi, Pampadromaeus barberenai, Bagualosaurus agudoensis, Gnathovorax cabreirai and Staurikosaurus pricei. All of these were relatively small herbivorous bipeds known as sauropodomorphs (which would later give rise to the giant sauropods), except Staurikosaurus, which was a meat-eating herrerasaurid, and Buriolestes, which was uniquely carnivorous among sauropodomorphs.

A close contender for this record, and some scientists would argue is equal to the Santa Maria Formation, are dinosaurs unearthed in the Ischigualasto Formation in the San Juan and Rioja provinces of north-west Argentina. The rocks here have been dated to c. 231.7 million years old, so in terms of geological time, the difference of 1.5 million years is negligible. Well-known early dinosaur species to have been found at this site include Herrerasaurus ischigualastensis and Eoraptor lunensis.

A potentially even older dinosaur - Nyasasaurus parringtoni - found in rocks dating to 243 million years old (during the Triassic's Anisian age) have been found in Tanzania, however due to very fragmentary remains, it is widely contested whether or not this creature was truly a dinosaur or some other form of reptile.