Mesozoic

Mesozoic
Name	Mesozoic
Color	Mesozoic
Top bar	all palaeozoic
Caption	A collage of iconic Mesozoic life: Tyrannosaurus, Triceratops, Pteranodon, Archaeopteryx, and Stegosaurus.
Timeline	Mesozoic
Lower boundary	251.902
Lower boundary method	Chronostratigraphic
Upper boundary	66.0
Upper boundary method	Chronostratigraphic
Lower def	First appearance of the conodont Hindeodus parvus
Upper def	Iridium-enriched layer associated with a major impact event
Previous	Paleozoic
Next	Cenozoic

Mesozoic. The Mesozoic Era is a major division of geologic time that followed the Paleozoic and preceded the Cenozoic. Spanning from approximately 252 to 66 million years ago, it is often called the "Age of Reptiles" due to the dominance of dinosaurs, pterosaurs, and marine reptiles. This era witnessed the breakup of the supercontinent Pangaea, dramatic shifts in climate, and the evolution of key modern groups, culminating in a catastrophic mass extinction.

Etymology and definition

The term "Mesozoic" derives from the Greek words *mesos* (middle) and *zōion* (animal), translating to "middle life." It was formally proposed in the 19th century by John Phillips, who divided the Phanerozoic Eon into three eras based on the fossil record. The era's boundaries are defined by two of Earth's most significant mass extinction events: it began in the wake of the Permian–Triassic extinction event and ended with the Cretaceous–Paleogene extinction event. These catastrophic boundaries mark profound transitions in the planet's biota.

Subdivisions (Triassic, Jurassic, Cretaceous)

The Mesozoic is subdivided into three geologic periods. The Triassic (252–201 million years ago) saw the recovery of life after the great Permian extinction and the emergence of the first dinosaurs and mammals. The Jurassic (201–145 million years ago) is famed for the proliferation of giant sauropods and the dominance of reptiles in air, sea, and on land. The Cretaceous (145–66 million years ago) was marked by the rise of flowering plants, advanced theropods like Tyrannosaurus rex, and the final chapter of non-avian dinosaurs.

Paleogeography and tectonics

At the era's dawn, all continental crust was amalgamated into the supercontinent Pangaea, surrounded by the global ocean Panthalassa. During the Late Triassic, Pangaea began to rift apart, a process driven by intense plate tectonics and mantle convection. By the Late Jurassic, the Central Atlantic had opened, separating Laurasia in the north from Gondwana in the south. The Cretaceous saw the continued breakup of Gondwana, opening the South Atlantic Ocean and isolating landmasses like India and Madagascar.

Climate and sea level

The Mesozoic climate was generally warmer than today, with no polar ice caps for most of the era. The Triassic was predominantly hot and arid, with vast interior deserts on Pangaea. The Jurassic became more humid, supporting lush conifer forests and high sea levels that created extensive shallow epicontinental seas like the Sundance Sea. The Cretaceous reached a climatic zenith, a "Greenhouse World" with very high atmospheric carbon dioxide levels, leading to peak sea levels that flooded continents and formed the Western Interior Seaway across North America.

Life in the Mesozoic

Terrestrial ecosystems were ruled by archosaurs, most notably the dinosaurs, which diversified into herbivorous ornithischians and carnivorous saurischians. The skies were occupied by pterosaurs and, later, the first birds like Archaeopteryx. Oceans teemed with reptiles like ichthyosaurs, plesiosaurs, and mosasaurs. The era saw the first true mammals, which remained small and nocturnal. Plant life evolved from cycad and fern-dominated floras in the Triassic to the first angiosperms (flowering plants) in the Cretaceous, revolutionizing terrestrial ecosystems.

End of the era: Cretaceous–Paleogene extinction event

The Mesozoic concluded abruptly with the Cretaceous–Paleogene extinction event approximately 66 million years ago. The primary cause is widely attributed to a massive asteroid impact at Chicxulub crater on the Yucatán Peninsula, evidenced by a global layer of iridium and shocked quartz. This cataclysm, possibly exacerbated by intense volcanism from the Deccan Traps, caused a rapid impact winter, collapsing food webs. It led to the extinction of all non-avian dinosaurs, pterosaurs, and most marine reptiles, paving the way for the rise of mammals and birds in the ensuing Cenozoic.

Category:Mesozoic Category:Geologic eras