What is the earliest known dinosaur?

Since the fossil record for dinosaurs is incomplete, we do not have fossils from the very first dinosaur that ever lived. Based on the age of the specimens that we do have, that first dinosaur may well have lived in the early Triassic Period. The earliest dinosaurs for which we do have well-documented fossils are found in Late Triassic Ischigualasto Formation in northern Argentina. Skeletons discovered in these rock layers include the meat-eating dinosaurs Herrerasaurus and Eoraptor, as well as the plant-eating dinosaur Pisanosaurus. Some rock layers from this formation are composed of ancient volcanic ash, which has yielded a radioisotopic age of 228 million years ago.