What was a key feature of the Triassic Period?

The Triassic Period, which spanned from approximately 252 to 201 million years ago, was marked by significant evolutionary developments, one of which was the rise of early mammals. During this time, the first true mammals emerged from the synapsid lineage, which had been present since earlier periods. These early mammals were generally small, nocturnal creatures that adapted to the new ecological niches forming after the Permian-Triassic extinction event.

The rise of mammals is particularly notable because it set the stage for the later diversification and dominance of mammals in subsequent geological periods, particularly after the extinction event at the end of the Cretaceous Period. Understanding this key feature provides insight into how life evolved following the mass extinctions that occurred prior to the Triassic and highlights the importance of this period in the history of vertebrate evolution.

Other options such as the emergence of flowering plants and the first appearance of birds relate more closely to later periods, with flowering plants becoming more prominent in the Cretaceous, and birds evolving from theropod dinosaurs in the Late Jurassic. The formation of major mountain ranges also predominantly occurred during later periods like the Mesozoic. Therefore, the rise of mammals is the defining characteristic of the Triassic.