Cretaceous

Cretaceous. The Cretaceous is a geologic period and system that spans from approximately 145 to 66 million years ago, constituting the final chapter of the Mesozoic Era. It is preceded by the Jurassic and followed by the Paleogene period of the Cenozoic Era. This interval is renowned for the dominance of dinosaurs on land, the emergence of modern flowering plants, and its catastrophic conclusion marked by the Cretaceous–Paleogene extinction event.

Etymology and history

The name "Cretaceous" is derived from the Latin *creta*, meaning "chalk," a direct reference to the extensive chalk deposits characteristic of this period, particularly in Western Europe. These deposits, formed from the skeletal remains of marine microorganisms like coccolithophores, were first studied in the Paris Basin and the white cliffs of Dover in England. The period was formally defined in 1822 by the Belgian geologist Jean d'Omalius d'Halloy, who identified it as the "Terrain Crétacé" while mapping the geology of France. The work of pioneering geologists like William Smith in Great Britain and Alcide d'Orbigny further established its stratigraphic significance within the broader Phanerozoic timescale.

Geology and subdivisions

The Cretaceous System is formally divided into two series: the Lower Cretaceous (or Early Cretaceous) and the Upper Cretaceous (or Late Cretaceous). These are further subdivided into twelve globally recognized stages, from the Berriasian at the base to the Maastrichtian at the top. Key stratigraphic boundaries are defined by Global Boundary Stratotype Sections and Points (GSSPs), such as the base of the Maastrichtian at Tercis-les-Bains in France. The period is marked by significant geological activity, including the continued breakup of the supercontinent Gondwana and the ongoing rifting of the Atlantic Ocean. Widespread marine transgressions created vast, shallow epicontinental seas like the Western Interior Seaway in North America and the Tethys Ocean across Eurasia.

Paleogeography and climate

During the Cretaceous, the continents continued to drift from the configuration of Pangaea towards their modern positions. The fragmentation of Gondwana led to the separation of South America from Africa and the isolation of India as it moved northward. The global climate was generally warmer and more equable than today, with high atmospheric carbon dioxide levels and no polar ice caps. Evidence from fossil flora, including finds from the Santana Formation in Brazil and the Cedar Mountain Formation in Utah, suggests lush, forested environments extended into high latitudes. Ocean circulation patterns, influenced by the expanding Atlantic Ocean and the shrinking Tethys Ocean, contributed to this greenhouse world, though some cooling trends are noted in the Maastrichtian.

Flora and fauna

The Cretaceous witnessed a major botanical revolution with the rapid diversification and rise to ecological dominance of the angiosperms (flowering plants), a transformation often called the "Cretaceous Terrestrial Revolution." This period was the zenith of dinosaur diversity, featuring iconic giants like the theropod Tyrannosaurus rex, the sauropod Argentinosaurus, and the ceratopsian Triceratops. The skies were ruled by advanced pterosaurs like Quetzalcoatlus and the earliest true birds, such as Ichthyornis. In the oceans, marine reptiles like mosasaurs and plesiosaurs were apex predators, while ammonites and belemnites flourished. Important fossil sites documenting this life include the Hell Creek Formation in Montana, the Yixian Formation in China, and the Kem Kem Beds in Morocco.

Extinction event

The Cretaceous concluded with one of the five major mass extinction events in Earth's history, the Cretaceous–Paleogene extinction event (K–Pg event). The primary cause is widely attributed to the catastrophic impact of a large asteroid at Chicxulub crater on the Yucatán Peninsula in Mexico, an event supported by a global layer of iridium-rich clay. This impact triggered widespread wildfires, tsunamis, and a prolonged "impact winter" that severely disrupted ecosystems. Concurrently, massive volcanic eruptions in the Deccan Traps of India likely contributed to environmental stress. The event resulted in the extinction of all non-avian dinosaurs, pterosaurs, ammonites, and most marine reptiles, paving the way for the rapid diversification of mammals and birds in the subsequent Paleogene period.

Category:Geological periods Category:Mesozoic