Dawn of the dinosaurs
In the arid badlands of San Juan Province lies one of the most complete records of the Triassic Period on Earth. The sedimentary rocks of the Ischigualasto Formation, dating to the late Carnian stage around 231 to 225 million years ago, preserve a record of the dawn of the dinosaurs. At that time, this region was not a desert. It was a volcanically active floodplain with rivers and a dense vegetation of ferns, horsetails, and giant petrified conifers of the species Protojuniperoxylon ischigualastianus, some of which grew over 40 meters tall.
This ancient environment supported a diverse fauna, but was not dominated by dinosaurs. The most abundant fossils found here belong to herbivorous reptiles. These include the beaked rhynchosaurs, like Hyperodapedon, and large, mammal-like cynodonts such as Exaeretodon argentinus. Dinosaurs and their immediate relatives were a minor part of this ecosystem, represented by small, nimble animals. Among the most significant discoveries are some of the earliest known dinosaurs, including Eoraptor lunensis and Herrerasaurus ischigualastensis. Eoraptor was a lightly-built carnivore about one meter in length, a fraction of the size of its later relatives.