Mesozoic

The Mesozoic Era, spanning about 252 to 66 million years ago, is the middle portion of the Phanerozoic Eon. It is divided into three periods: Triassic, Jurassic, and Cretaceous. The era is commonly nicknamed the Age of Reptiles due to the dominance of dinosaurs, which varied in form and ecology, from herbivorous to apex carnivores, and the appearance of the first birds. Mammals were present but generally small and ecologically secondary.

Geography and climate: The early Mesozoic followed the Permian extinction, with warm global climates and levels of atmospheric CO2 higher than today. The supercontinent Pangaea began to break apart in the Triassic, producing rift valleys and new ocean basins; by the Late Cretaceous, continental positions were more separated and sea levels fluctuated, creating inland seas.

Biology and evolution: The Triassic saw the rise of dinosaurs and archosaurs and the first true mammals. The Jurassic witnessed diversification of dinosaurs and the appearance of early birds; marine reptiles such as ichthyosaurs and plesiosaurs thrived. The Cretaceous saw further evolution of dinosaurs, flowering plants (angiosperms) emerged and diversified, and ammonites and reef-building corals thrived.

Extinction and legacy: The end-Triassic mass extinction cleared ecological niches, and the end-Cretaceous event, likely caused by a large asteroid impact and volcanic activity, led to the extinction of most non-avian dinosaurs and many marine reptiles, marking the transition to the Cenozoic.