Herrerasaurs are among the oldest known dinosaurs, appearing in the fossil record about 228 million years ago (mid-Triassic). They became extinct by the end of the Triassic period. Herrerasaurs were small to medium-sized carnivores generally resembling theropods, which may be their closest relatives. The best known representatives of this group are from South America (Brazil, Argentina), where they were first discovered in the 1960s. A nearly complete skeleton of Herrerasaurus ischigulastensis was discovered in the Ischigualasto Formation in San Juan, Argentina, in 1988. Less complete herrerasaurians have been found in North America, and they may have inhabited other continents as well.