The Triassic Period. 251-200 Ma.
The first of the three periods of the Mesozoic Era, preceding the following Jurassic Period and Cretaceous Period of the Mesozoic Era, while following on after the previous Permian Period, the last of the six periods of the earlier Palaeozoic Era.
Known for its single, lone supercontinent, Pangaea, as well as intermediate climates and ecological conditions, compared to the very different conditions when looking at the earlier Permian Period, and the later Jurassic Period, in cross-comparison - with the Triassic being a long period of recovery and change. This was after the terrible ravages of the end-Permian Mass Extinction, known also as "The Great Dying" to Palaeontologists.
With over 95% of life wiped out at the end of the Permian Period, the early Triassic was a time of ecological recovery, slowly rebuilding from the worst extinction of all time. The niches were, earlier in the period, filled by a smaller number of briefly, wildly successful animal and plant species. Ultimately, these were soon faced with greater competition, as life on Earth got back on its feet again, and animals such as Lystrosaurus, were not quite as widespread and ecologically dominant as they were at the beginning of the period. With greater recovery, came greater diversity.
Major reptile groups did survive the Great Dying, and among them, were indeed the ancient mammal-like reptiles, as well as the then relatively very new Cynodonts, and the older Dicynodonts - to which the very early Triassic Lystrosaurus and the very late Triassic Placerias, both belong. However, another reptile group survived the Great Dying; the Archosaurians.
While it is true that older reptile groups would dominate the Triassic for well over half of it, the truth is, the Archosaurs arose from minor, middling ecological status (beginning in the late Permian, and just about getting through the Great Dying, as relatively middling, middle of the food chain, dog sized animals in the original Archosauria species) to then outshine all other reptile types, towards the end of the Triassic. First establishing successful dominance over some ecological niches, in some regions, by as early as the end of the Early Triassic in some cases, they took over the world - in the air as well as on the land - by the end of the Triassic Period. The Archosaurians, in their main derived dynasties of Pterosauria, Crocodylia/Eosuchia, and of course, Dinosauria, ruled.