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The Mesozoic Era is an interval of geological time from about 250 million years ago to about 65 million years ago. It is called the Age of Dinosaurs because most dinosaurs developed, and went extinct, during that time. The Chicxulub impact and other events ended the era when a majority of species on earth went extinct.

It is one of three geologic eras of the Phanerozoic eon. The division of time into eras dates back to Giovanni Arduino, in the 18th century, although his original name for the era now called the "Mesozoic" was "Secondary" (making everything after, including the modern era, the "Tertiary"; the current term Quaternary was later proposed for the modern era, following the same numbering principle). Lying between the Paleozoic and the Cenozoic, "Mesozoic" means "middle life", deriving from the Greek prefix meso-/μεσο- for "between" and zoon/ζωον meaning "animal" or "living being". It is often called the "Age of the Reptiles", after the dominant fauna of the era.

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 drifting provided for speciation and other important evolutionary developments. 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.


Geologic periods

Following the Paleozoic, the Mesozoic extended roughly 180 million years: from 251 million years ago (Mya) to when the Cenozoic era began 65 Mya. This time frame is separated into three geologic periods. From oldest to youngest:

The lower (Triassic) boundary is set by the Permian-Triassic extinction event, during which approximately 90% to 96% of marine species and 70% of terrestrial vertebrates became extinct. It is also known as the "Great Dying" because it is considered the largest mass extinction in the Earth's history. The upper (Cretaceous) boundary is set at the Cretaceous-Tertiary (KT) extinction event (now more accurately called the Cretaceous–Paleogene (or K–Pg) extinction event[1]), which may have been caused by the impactor that created Chicxulub Crater on the Yucatán Peninsula. Approximately 50% of all genera became extinct, including all of the non-avian dinosaurs.

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