The Triassic world emerged after the end-Permian mass extinction which almost wiped out life on earth. During the Triassic era, the world's reptiles first appeared.
252 million years ago, the world was deserted and dying. Volcanic eruptions left molten rock covering more than 770 thousand square miles. This released carbon dioxide and greenhouse gases. This era's extreme climate changes occurred relatively quickly. Seas became so acidic that shelled creatures could no longer grow. Oxygen levels were so low that many animals could no longer breathe on land. Earth neared complete extinction.
No animal, or human for that matter, could have prepared for such devastation. The few lucky survivors already had adapted to survive. These few migrated into an empty world that lacked all the creatures it spent millions of years perfecting.
Before the extinction, our distant ancestors, the protomammals, ruled the world. They came in all shapes and sizes from the saber-fanged Gorgonosids, to its prey the barrel-bodied Dicynodonts, or the smaller Cynonts. But when the Triassic period began they lost all their power.
Who overpowered the mighty protomammals? Reptiles. During the Permian most reptiles were small and lizard-like. But with the climate change and the removal of so many predators, these species became rulers. Eventually, they bowed down to the dinosaurs.