What major biological event occurred during the Mesozoic Era?

The Mesozoic Era, often referred to as the "Age of Reptiles," is particularly significant for the re-establishment and diversification of biological life following the massive Permian-Triassic extinction event that occurred at the end of the Paleozoic Era. This era marks a time when dinosaurs became the dominant terrestrial vertebrates, alongside the emergence of various other life forms, including early mammals and birds. The vast array of ecosystems that developed during the Mesozoic contributed to a flourishing of biodiversity, making this era critical for the subsequent evolutionary history of life on Earth.

While the appearance of large mammals occurred later, predominantly in the Cenozoic Era, and the development of insects predates the Mesozoic, the re-establishment of biological diversity is a defining feature of this era. It signifies the recovery and adaptation of life after the previous mass extinction, setting the stage for future evolutionary developments. The formation of the first land plants is also an important event but is more closely associated with the earlier Paleozoic Era.