The Mesozoic era extends from 248 to 65 million years ago, and is affectionately known as the Era of the Dinosaurs. It is divided into three periods: Triassic, Jurassic and Cretaceous.
It begins after the greatest mass extinction in history, the Permian-Triassic extinction, which was probably caused by an asteroid 3 to 7 miles wide colliding with the Earth, and in which 90% of ocean life and 70% of land-based life on Earth perished. The first mammals, and the first primitive dinosaurs, also known as Thecodonts, began to appear at the beginning of the Triassic period, which ended with another, smaller mass extinction in which roughly 35% of land animal species and most of the marine reptiles died out. This was partly responsible for the success of the dinosaurs during the remainder of the Mezozoic era, as it allowed them to expand into new environmental niches.
The first birds, bees, flowering plants (Angiosperms), ants and other insects appeared during the Mesozoic era, and these have endured to become counted among the most successful modern species. Mammals did far less well during this time, with only the marsupials (for example, the modern-day oppossum) becoming common. To visualize the landscape during much of the Mesozoic era, think of vast forests of cycads, ferns and gymnosperms as large or larger than modern-day trees.
As is now widely-known, this era ended 65 million years ago with another mass extinction which effectively eliminated the dinosaurs, along with 50% of invertebrate marine species. It is thought that this extinction was caused by an asteroid impact or supervolcanic activity.
Sources:
http://www.horseshoecrab.org/evo/mezo/mezo.html
http://www.mhhe.com/biosci/genbio/tlw/full/instructor/teachtech/di/di14.html
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