Cretaceous

The Cretaceous is defined by the International Commission on Stratigraphy as the period from 145.0 to 66.0 million years ago, following the Jurassic and preceding the Paleogene. It is divided into two epochs: the Early Cretaceous (145–100.5 Ma) and the Late Cretaceous (100.5–66 Ma), each further split into stages such as the Albian and Maastrichtian. The base of the period is marked by the appearance of the ammonite species Berriasella jacobi, though the exact boundary is debated. The name derives from the Latin creta, meaning chalk, reflecting the extensive chalk deposits formed in the Late Cretaceous, such as the White Cliffs of Dover. These deposits are composed largely of coccolithophore remains, indicating warm, shallow seas.

During the Cretaceous, the supercontinent Pangaea continued to break apart, with the Atlantic Ocean widening and the Indian Ocean opening. Sea levels were exceptionally high, flooding continental interiors and creating epicontinental seas that deposited thick marine sediments. The climate was among the warmest in Earth's history, with no polar ice caps and tropical conditions extending to high latitudes; deep-ocean temperatures may have reached 15–20°C. This greenhouse state was driven by elevated atmospheric CO2 from volcanic activity, particularly from the Deccan Traps and large igneous provinces. Oceanic anoxic events, such as the Cenomanian-Turonian event, caused widespread oxygen depletion and organic-rich black shale deposition, which are important source rocks for petroleum.

The Cretaceous saw the peak of dinosaur diversity, including iconic forms like Tyrannosaurus rex and Triceratops, alongside marine reptiles such as mosasaurs and plesiosaurs. Flowering plants (angiosperms) first appeared and rapidly diversified, replacing gymnosperms as the dominant vegetation by the end of the period. Insects co-evolved with these plants, leading to pollination mutualisms. In the oceans, rudist bivalves formed extensive reefs, and foraminifera and ammonites thrived. The period also witnessed the rise of early mammals, which remained small and nocturnal. The end of the Cretaceous is defined by the Chicxulub impact, which triggered a mass extinction that eliminated ~75% of species, including all non-avian dinosaurs, paving the way for mammalian dominance.

Beyond the headline events, the Cretaceous holds many niche details. The period saw the evolution of the earliest ants and termites, which became major ecological players. The fossil record includes the oldest known bird, Archaeopteryx, though it dates to the Late Jurassic; Cretaceous birds like Confuciusornis show advanced flight capabilities. The Cretaceous also featured the largest known flying animals, the pterosaurs, with Quetzalcoatlus reaching wingspans of over 10 meters. The period's chalk deposits are not just geological curiosities; they are quarried for cement and were used in medieval manuscripts. Additionally, the Cretaceous is a key interval for understanding the evolution of plate tectonics, as the opening of the South Atlantic is precisely dated by magnetic anomalies. The Deccan Traps, a massive volcanic province in India, may have contributed to the extinction, but its exact role remains debated.

Cretaceous–Paleogene extinction event

A mass extinction ~66 million years ago, likely caused by an asteroid impact, that killed off non-avian dinosaurs.

Angiosperms

Flowering plants, which first appeared and diversified during the Cretaceous.

Oceanic anoxic event

Periods of severe oxygen depletion in the oceans, leading to black shale deposition.

Chicxulub impact

The asteroid impact in present-day Mexico that is linked to the end-Cretaceous extinction.