Mesozoic - Geologic Periods

Geologic Periods

Following the Paleozoic, the Mesozoic extended roughly 180 million years: from 252.2 to 66 million years ago when the Cenozoic Era began. This time frame is separated into three geologic periods. From oldest to youngest:

• Triassic (252.2 to 201.3 million years ago)
• Jurassic (201.3 to 145 million years ago)
• Cretaceous (145 to 66 million years ago)

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), which may have been caused by the impactor that created Chicxulub Crater on the Yucatán Peninsula. Towards the Late Cretaceous large volcanic eruptions are also believed to have contributed to the Cretaceous–Paleogene extinction event. Approximately 50% of all genera became extinct, including all of the non-avian dinosaurs.