What is the Mesozoic era (Phanerozoic Eon)?

The Mesozoic Era was the time from 248 million to 65 million years ago, so it lasted more than 180 million years. During the Mesozoic, the Earth was very different than it is now. The climate was warmer, the seasons were very mild, the sea level was higher, and there was no polar ice. At the beginning of the Mesozoic, all of the world's continents were joined into the supercontinent of Pangea, which rifted into Laurasia in the north and Gondwanaland in the south.  By the end of the era most of continents had separated into their present form.

Toward the beginning of the Mesozoic Era there was a depleted ecosystem world-wide. Many of the old life forms had just gone extinct in the Permian Extinction, the world's largest mass extinction. This depleted state was followed by an explosion of new life forms, which included the dinosaurs and mammals, and later in the Mesozoic, the birds and flowering plants.

During this time, many modern forms of plants, invertebrates, and fishes evolved. On land, dinosaurs were the dominant animals, while the oceans were populated by large marine reptiles and many other invertebrate groups, and Pterosaurs ruled the air. Plants included ferns and gymnosperms. Mammals were small and insignificant, but probably numerically common. For most of this period, the climate worldwide was warm and tropical, and shallow seas covered low-lying landmasses.  

The Mesozoic Era is divided into three periods, each lasting many millions of years: the Triassic, Jurassic, and Cretaceous. The Triassic saw the emergence of many modern invertebrate groups, and on land the archosaur reptiles replaced the therapsids. In the oceans Ichthyosaurs such as Shonisaurus became as large as whales. The Jurassic was the height of the dinosaur era, with giants such as Brachiosaurus, Stegosaurus, etc, and mammals tiny and shrew-like. Distinctive plants like ferns, Cycads, Bennettitales, and Cheirolepidiaceae conifers characterized the landscape. During the Cretaceous period, the first flowering plants appeared, birds and fish diversified, and new types of dinosaurs appeared. The climate cooled and unique dinosaurs evolved on different continents. 

The Mesozoic era came to an end with the great terminal extinction event known as the K-T (Cretaceous-Tertiary) event.