Overview

The Cretaceous is the last period of the Mesozoic era and represents a major chapter in Earth history roughly between 145 and 66 million years ago. It extended for tens of millions of years and bridged the world of Jurassic ecosystems to the modern patterns that followed in the Cenozoic. The period is commonly subdivided into two major intervals often called the Lower (Early) and Upper (Late) Cretaceous.

Chronology and subdivisions

The Cretaceous is split into two epochs. The Lower (or Early) Cretaceous begins near the start of the period and the Upper (or Late) Cretaceous ends with the famous extinction event that closed the Mesozoic. Because stratigraphy and geologic stage names vary by region and with successive refinements, study of the period often refers to regional formations, sea-level cycles, and fossil assemblages to establish finer resolution.

Geography, sea level, and sedimentation

Global plate arrangements during the Cretaceous produced extensive shallow seas and epicontinental seaways. Sea levels were generally much higher than present at many times in the period, flooding continental interiors and creating environments where marine sediments accumulated over broad areas. In these warm, shallow waters large deposits of carbonate sediments formed, producing thick sequences of limestone and the distinctive chalk deposits known from places such as northwest Europe.

Life and ecosystems

The Cretaceous hosted diverse terrestrial and marine life. On land, dinosaurs remained dominant, with groups such as tyrannosaurs, ceratopsians, hadrosaurs, and many others radiating into new niches. The period is also notable for the diversification and ecological rise of flowering plants (angiosperms), which began to change terrestrial food webs and plant–pollinator relationships. In the seas, ammonites, marine reptiles, and abundant planktonic organisms—including coccolithophorids whose calcareous plates helped form chalk—were important components of marine ecosystems.

Chalk, coccoliths, and sedimentary records

Chalk is a fine-grained limestone largely composed of the microscopic calcium carbonate plates (coccoliths) produced by single-celled algae. When these organisms flourished in great numbers, their remains accumulated on seafloors to build the chalk beds preserved in the rock record. Such deposits provide a detailed archive of ocean chemistry, climate, and biological productivity during parts of the Cretaceous.

End‑Cretaceous extinction and legacy

The close of the Cretaceous is marked by a major mass extinction that removed many dominant groups from Earth ecosystems. A combination of catastrophic events is implicated: an impact near the Yucatán Peninsula and extensive volcanic activity in regions such as the Deccan Traps have been linked to abrupt environmental change. The extinction cleared ecological space for the Cenozoic rise of mammals, modern birds, and reconfigured plant communities, fundamentally reshaping life on Earth.

These topics can provide focused entry points to sedimentary records, fossil groups, and the events that make the Cretaceous a pivotal period in Earth's deep past.