Dinosaurs did not appear until the Middle to Late Triassic. But many other animals lived in the Triassic world. The therapsids were mammallike reptiles that ruled the earth before dinosaurs. They included the labyrinthodonts, the late survivors of the early amphibians; procolophonids, the early plant-eating reptiles; dicynodonts, the turtlebilled plant-eaters; and cynodonts, the ancestors of mammals.
Canadian Museum of Nature
Lystrosaurus, a dog-size dicynodont, was the ruling animal at the beginning of the Triassic. Scientists have found its fossils on almost every continent of the world-Asia, Africa, South America, and Antarctica. There were no land or weather barriers to stop animals from roaming freely. Other therapsids were also present in the Early Triassic, including the cynodont Thrinaxodon.
Archosaurs first appeared in the Late Permian as a minor part of the fauna. Dinosaurs, crocodiles, and pterosaurs all descended from the earliest archosaurs. The crocodile-like proterosuchians were the first archosaurs, and they gave rise to many other groups, including the aquatic phytosaurs, the terrestrial carnivorous groups Erythrosuchidae and Rauisuchidas, and the herbivorous aetosaurs. Proterosuchus, which dates back to the early Triassic, was almost the size of a modern crocodile.
Halfway through the Early Triassic, the Lystrosaurus fauna was replaced by other dicynodonts, cynodonts, and archosaurs. Important archosaurs that appeared were Erythrosuchus, a large meat-eater; and Euparkeria, a fast, small meat-eater. Euparkeria was important because it had features of later archosaurs; it stood more upright, it had armor, and it had no teeth on its palate. Also appearing around this time were the rhynchosaurs. This group of land herbivores were close relatives of the archosaurs and were among the most plentiful plant-eaters in the Middle and early Late Triassic.
The Middle Triassic is important as a time of transition. The archosaurs were becoming more important and more meat-eaters were emerging, including many kinds of small, crocodile-like archosaurs. Also showing up were the large, four-legged predators, the Prestosuchidae and Rauisuchidae, and the small land carnivore Gracilisuchus. All these were members of the line leading to crocodiles.
The ancestors of the dinosaurs were beginning to be important. Lagosuchus, Pseudolagosuchus, and Lagerpeton were tiny meat-eaters with long legs. They were close to the common ancestors of dinosaurs and pterosaurs, with ankles like later dinosaurs.