The Triassic rocks of the Ischigualasto formation of Argentina are thought to be some of the richest fossil deposits, and include the earliest known, well-preserved fossils of such dinosaurs as Herrerasaurus ischigualastensis, Eoraptor lunensis, and Pisanosaurus mertii. The drab, gray rocks of this formation were deposited in a humid environment. The area where this formation is found—the Ischigualasto Valley, or the “Valley of the Moon”—is located to the east of a subduction zone. Consequently, it also received deposits of volcanic ash during the Triassic. Dating of this ash has enabled scientists to determine an age of approximately 228 million years for this formation, placing it in the Middle to Late Triassic periods.

Vertebrate land fossils are abundant in the formation, including skeletons of advanced forms of synapsids, and odd beaked reptiles called rhynchosaurs. There are also large carnivores called rauisuchians, and smaller archosaurs, but the most prevalent fossils in the formation are of dinosaurs. In fact, based on these dinosaur fossils and ones found in the Santa Maria formation of Brazil, some scientists suggest that perhaps dinosaurs arose in South America.