The Mesozoic Era follows the most devastating mass extinction in earth's history. The marine biosphere is rebuild following extinctions in most of the major Paleozoic marine groups. The new fauna is dominated by molluscs, fish and corals. Reptiles also radiate into the marine realm, becoming the first large air-breathing predators.
On land, the diapsid reptiles replace the synapsids that dominated the Permian. By the end of the Triassic, synapsids play only bit parts. At the start of the Jurassic, while living on the fringes and grubbing out an existence, they evolve into the mammals.
After the Triassic, big changes occur. Diapsid reptiles radiate into pterosaurs and a variety of dinosaur forms. The Jurassic is dominated by giant herbivorous sauropods, while herbivorous ornithischians and carnivorous theropods also abound. In the late Jurassic, birds evolve from the theropods. Dinosaurs continue their dominance in the Cretaceous, with the ornithischians and theropods becoming more important.
The end of the Mesozoic is marked by the a sudden, massive extinction event. The second largest extinction in earth's history happens quite suddenly. Data collected within the last 20 years, strongly suggests that an asteroid impact on the earth played a major role in the Cretaceous-Tertiary extinctions.