The Mesozoic Era is referred to as the "Age of Reptiles". During the Mesozoic, reptiles inhabited the land, the seas, and the air. The Mesozoic is the time in which the dinosaurs lived.
The Mesozoic is also the time in which mammals and birds first appeared on Earth.
Major advances in plants also occurred during the Mesozoic. The flowering plants or angiosperms first appeared in the Mesozoic and were widespread by the Middle Cretaceous.
At the beginning of the Mesozoic Era, diversity (as indicated by the number of genera) was low, following the Permian extinctions. Recovery from the Permian extinctions was slow for many groups. In the oceans, the molluscs re-expanded to become much more diverse than in the Paleozoic, and modern reef-building corals, swimming reptiles, and new kinds of fishes appreared.
Diversity of marine animals through geologic time, as indicated by number of known fossil genera.
A mass extinction occurred at the end of the Triassic Period. The Triassic extinction affected life on the land and in the sea, causing about 20% of all marine animal families to become extinct. Animals that died out completely in the Triassic extinction included conodonts and placodonts (reptiles). Also affected were most species of bivalves, ammonoids, plesiosaurs, and ichthyosaurs, although these groups recovered and rediversified in the Jurassic. Among the terrestrial organisms affected by the extinction were mammal-like reptiles and large amphibians.
Diversity increased in the Jurassic, and rose quickly during the Cretaceous to higher levels than had existed previously. Approximately 2500 genera of marine animals existed during the Late Cretaceous, which is well above the relatively constant level of maximum diversity that existed during the Paleozoic (1000 to 1500 genera).Much of this expansion in diversity was related to the appearance of new types of marine predators, including advanced teleost fishes, crabs, and carnivorous gastropods. The decline of organisms which lived attached to the seafloor (such as brachiopids and stalked crinoids) may be in part related to the increase in predators in the Cretaceous seas.
Life in the Cretaceous consisted of a mixture of both modern and ancient forms. For example, modern types of bivalves, gastropods, and fishes were present along with now-extinct organisms such as ammonoids, belemnoids, and marine reptiles. On the land, the predominant plants changed from gymnosperms (which dominated the earlier Mesozoic) to angiosperms (flowering plants).
A major extinction event occurred at the end of the Cretaceous Period, affecting both vertebrates and invertebrates, on land and in the sea. This is the extinction event which caused the disappearance of the dinosaurs, ammonoids, large marine reptiles (plesiosaurs and mosasaurs), and the rudists (reef-forming bivalves), among other groups. There were also drastic reductions in coccolithophores, planktonic foraminifera, radiolarians, and belemnoids.