THE Mesozoic ERA is one of three geologic eras of the Phanerozoic eon. Lying between the Paleozoic and the Cenozoic these three eras comprise the Pha-nerozoic eon. The Mesozoic was a time of tectonic, climatic, and evolutionary activity. The continents gradually shifted from a state of connectedness into their present configuration. The climate was exceptionally warm throughout the period, also playing an important role in the evolution and diversification of new animal species. By the end of the era, the basis of modern life was in place. The Mesozoic era extended roughly 180 million years, from 251 million years ago (Mya) to when the Cenozoic era began 65 Mya. This Era is further separated into three geologic Periods. From oldest to youngest: Triassic (251 Mya to 199 Mya), Jurassic (199 Mya to 145 Mya), Cretaceous (145 Mya to 65 Mya).
The lower (Triassic) boundary is set by the Perm-ian-Triassic extinction, during which approximately 90-96 percent of marine species and 70 percent of terrestrial vertebrates became extinct. It is also known as the Great Dying because it is considered the largest mass extinction in history. The upper (Cretaceous) boundary is set at the Cretaceous-Tertiary (KT) extinction, which may have been caused by the meteor that created the Chicxulub Crater on the Yucatán Peninsula. Approximately 50 percent of all genera became extinct, including all of the non-avian dinosaurs.
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