Mesozoic

EtymologyName formalityFormalNickname(s)Age of Reptiles, Age of Dinosaurs, Age of ConifersUsage informationCelestial bodyEarthRegional usageGlobal (ICS)Time scale(s) usedICS Time ScaleDefinitionChronological unitEraStratigraphic unitErathemTime span formalityFormalLower boundary definitionFirst appearance of the Conodont Hindeodus parvus.Lower boundary GSSPMeishan, Zhejiang, China
31°04′47″N119°42′21″E / 31.0798°N 119.7058°E / 31.0798; 119.7058Lower GSSP ratified2001Upper boundary definitionIridium enriched layer associated with a major meteorite impact and subsequent K-Pg extinction event.Upper boundary GSSPEl Kef Section, El Kef, Tunisia
36°09′13″N8°38′55″E / 36.1537°N 8.6486°E / 36.1537; 8.6486Upper GSSP ratified1991

The Mesozoic Era is an era of Earth's geological history, lasting from about 252 to 66 million years ago, comprising the Triassic, Jurassic and CretaceousPeriods. It is characterized by the dominance of archosaurian reptiles such as the dinosaurs, and of gymnosperms such as cycads, ginkgoaceae and araucarian conifers; a hot greenhouse climate; and the tectonic break-up of Pangaea. The Mesozoic is the middle of the three eras since complex life evolved: the Paleozoic, the Mesozoic, and the Cenozoic.

The Mesozoic is commonly known as the Age of the Dinosaurs because the terrestrial animals that dominated the majority of it were dinosaurs. This era began in the wake of the Permian–Triassic extinction event, the largest mass extinction in Earth's history, and ended with the Cretaceous–Paleogene extinction event, another mass extinction whose victims included the non-avian dinosaurs, pterosaurs, mosasaurs, and plesiosaurs. The Mesozoic was a time of significant tectonic, climatic, and evolutionary activity. The supercontinent Pangaea began to break apart into separate landmasses. The climate of the Mesozoic was varied, alternating between warming and cooling periods. Overall, however, the Earth was hotter than it is today.

Dinosaurs first appeared in the Mid-Triassic, and became the dominant terrestrial vertebrates in the Late Triassic or Early Jurassic, occupying this position for about 150 or 135 million years until their demise at the end of the Cretaceous. Archaic birds appeared in the Jurassic, having evolved from a branch of theropod dinosaurs, then true toothless birds appeared in the Cretaceous. The first mammals also appeared during the Mesozoic, but would remain small—less than 15 kg (33 lb)—until the Cenozoic. Flowering plants appeared in the Early Cretaceous and would rapidly diversify through the end of the era, replacing conifers and other gymnosperms (sensu lato), such as ginkgoales, cycads and bennettitales as the dominant group of plants.