Which group of organisms rapidly diversified during the Mesozoic Era?

During the Mesozoic Era, which spanned from about 252 to 66 million years ago, reptiles underwent significant diversification. This period is often referred to as the "Age of Reptiles," marked by the emergence and evolution of various groups, including dinosaurs, crocodilians, and marine reptiles. The Mesozoic provided numerous ecological niches due to changes in climate, geography, and available habitats, allowing reptiles to evolve into many forms, from flying pterosaurs to the gigantic sauropods.

Furthermore, reptiles were able to exploit terrestrial environments effectively, leading to their dominance during this era. While other groups, such as mammals and birds, also existed during the Mesozoic, they did not diversify to the same extent as reptiles during this period. Instead, mammals were primarily small and remained relatively insignificant compared to the larger, more dominant reptilian groups. Birds, which evolved from theropod dinosaurs towards the end of the Mesozoic, also reflect a later development in the diversification timeline. Thus, reptiles are the most representative group for rapid diversification throughout the Mesozoic Era.